The sea is the connected body of salt water that covers 70 percent of the Earth's surface. The sea is important in moderating the Earth's climate, in providing food and oxygen, in its enormous diversity of life, and for navigation. The study of the sea is called oceanography. The sea has been travelled and explored since ancient times, but its scientific study dates broadly from the voyages of Captain James Cook to explore the Pacific Ocean between 1768 and 1779.
Seawater is characteristically salty. The main solid in solution is sodium chloride but the water also contains chlorides of potassium and magnesium, alongside many other chemical elements, in a composition that hardly varies across the world's oceans. However the salinity varies quite widely, being lower near the surface and near the mouths of large rivers and higher in the cold depths of the ocean. The sea surface is subject to waves caused by winds. Waves decelerate and increase in height as they approach land and enter shallow water, becoming tall and unstable, and breaking into foam on the shore. Tsunamis are caused by submarine earthquakes or landslides and may be barely noticeable out at sea but can be violently destructive on shore. Winds create currents through friction, setting up slow but stable circulations of water throughout the sea. The directions of the circulation are governed by several factors including the shapes of the continents and the rotation of the earth. Complex deep sea currents known as the global conveyor belt carry cold water from near the poles to every ocean. Large-scale movement of seawater is also caused also by the tide, the twice-daily rhythm of the gravitational pull exerted by the Moon, and to a lesser extent by the Sun, on the Earth. Tides may have a very high range in bays or estuaries such as the Bay of Fundy where tidal flows are funnelled into narrow channels.
All the major groups of living organisms are found in the sea including bacteria, protists, algae, plants, fungi and animals. It is widely regarded to be the place where life first began, as well as where many of the major groups evolved. The sea contains a wide range of habitats and ecosystems, ranging vertically from the sunlit surface waters and the shoreline to the enormous depths and pressures of the cold, dark abyssal zone, and in latitude from the waters under the Arctic ice to the colourful diversity of coral reefs in tropical waters. It provides substantial supplies of food, mainly fish, but also of other animals including whales and of seaweeds to people around the world, both of wild-caught fish and from aquaculture.
Human uses of the sea include trade, food production, leisure activities such as swimming, sailing and scuba diving, mineral extraction, and warfare. Many of these activities also create pollution. The sea is important in human culture, with major appearances in literature since Homer's Odyssey, in marine art, in cinema and theatre, and in classical music. Symbolically, the sea appears as monsters such as Scylla in mythology, and represents the collective unconscious in some forms of psychotherapy.
The sea covers fully 70 percent of the Earth's surface with liquid water. Seen from space, our planet appears as a "blue marble" of various forms of water: salty oceans, sea ice, clouds. The science fiction author Arthur C. Clarke once suggested that "Earth" should have been named "Ocean" as the sea is its dominant feature, both critically important to life on earth, and recently strongly affected by human activities such as overfishing and pollution.
The sea is from one point of view the World Ocean, the interconnected system of all the Earth's oceanic waters. About 97.2 percent of the Earth's water is found in the seas, some 326 million cubic miles (1360 million cubic kilometres) of salty water. Of the rest, 2.15 percent is accounted for by ice in glaciers, surface deposits and sea ice and 0.65 percent is in the form of vapour or liquid fresh water in lakes, rivers, the ground and air.
"Freedom of the seas" is a principle in international law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in international waters. Today, this concept is enshrined in the United Nations Convention on the Law of the Sea which came into force in 1994. Article 87(1) states: "The high seas are open to all states, whether coastal or land-locked." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying of submarine cables, building artificial islands, fishing and scientific research. Territorial waters extend to 12 nautical miles (22 kilometres; 14 miles) from the coastline and in these waters, the coastal state is free to set laws, regulate use and exploit any resource.
Sea, ocean, lake 
Next to its use in "the sea" as designating the totality of interconnected bodies of water also known as the World Ocean, the word "sea" can also be used for specific, much smaller bodies of water, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas) or wholly (as inland seas) bordered by land. However, the Sargasso Sea has no coastline and lies within a circular current, the North Atlantic Gyre. It is a distinctive body of water with brown Sargassum seaweed and calm blue water, very different from the rest of the Atlantic Ocean. Seas are generally larger than lakes, and contain salt water rather than freshwater, but some geographic entities known as "seas" are enclosed inland bodies of water that are not salty: for instance, the Sea of Galilee is a freshwater lake.[a] The Law of the Sea states that all of the ocean is "sea".[b]
A characteristic of seawater is that it is salty. It is usual to measure salinity in parts per thousand and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35ppt. The Mediterranean Sea is slightly higher at 37ppt and the Dead Sea has as much as 300 grams (11 oz) dissolved solids per litre (300ppt). Although sodium chloride is the main salt present, constituting about 85 percent of the solids in solution, there are also 5 grams (0.18 oz) per litre of the chlorides of other metals such as potassium and magnesium and 3 grams (0.11 oz) of sulphates, carbonates, bromides and other salts. A kilogram (2.2 lb) of salt can thus be found in 28 litres or one cubic foot of typical ocean water. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is very stable throughout the world's oceans.
The circumstances which cause the salinity of a body of water to vary include evaporation from its surface (increased by high temperatures, wind and wave motion), precipitation on its surface, the freezing or melting of sea ice, the melting of glaciers, the influx of fresh river water, and the mixing of bodies of water of different salinities. The Baltic Sea for example is in a cool climatic region with low evaporation, has many rivers flowing into it and intermittent replenishment from the open ocean. The occasional influx of water from the North Sea creates a cold, dense under layer that hardly mixes with the surface layers. The uppermost layer may have a salinity of 10 to 15ppt, with even lower levels in the estuaries. The Red Sea experiences high atmospheric temperatures causing high evaporation but little precipitation, few rivers flow into it and the Bab-el-Mandeb, joining it to the Gulf of Aden, is narrow. Its salinity is high and averages 40ppt.
The temperature of the sea is dependent on the amount of solar radiation falling on the surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is about −2 °C (28 °F). Cold water is denser than warm water and tends to sink. There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.
Seawater has a freezing point of about −1.8 °C (28.8 °F). When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions this freezes into a thin flat sheet known as nilas which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join together into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped in the interstices between the ice crystals. Nilas may have a salinity of 12–15 ppt but by the time the sea ice is one year old, this has fallen to 4–6 ppt. The air remains and may support an ice ecosystem which includes viruses, bacteria, protozoa, algae and fungi.
The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton, but also include some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen which dissolves in the seawater where it is used by marine animals. At night, photosynthesis stops and the amount of dissolved oxygen declines. In the deep sea where insufficient light penetrates for plants to grow, there is very little dissolved oxygen.
Seawater is slightly alkaline and during historic times has had a pH of about 8.2. More recently, increased amounts of carbon dioxide in the atmosphere have resulted in more of it dissolving in the ocean forming carbonic acid and has raised this pH level to 8.1. The pH is expected to reach 7.7 by the year 2100, an increase of 320 percent in acidity in a century. One important element for the formation of skeletal material in marine animals is calcium but it is easily precipitated out in the form of calcium carbonate as the sea becomes more acid. This is likely to have profound effects on certain planktonic marine organisms because their ability to form shells will be reduced. These include snail-like molluscs known as pteropods, single-celled algae called coccolithophorids and foraminifera. All of these are important parts of the food chain and a diminution in their numbers will have significant consequences. In tropical regions, corals are likely to be severely affected by a lack of calcium with knock-on effects for other reef dwellers.
The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 metres (3,300 ft) under ideal conditions. In reality, there is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres (660 ft). The Blue Grotto at Capri demonstrates this absorption of short wave light. Red is selectively removed leaving the cavern bathed in a blue glow from light admitted through a large underwater entrance.
Wind blowing over the surface of a body of water forms waves. The friction between air and water caused by a gentle breeze on a pond causes ripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they travel nearly matches the speed of the wind. The waves form at right angles to the direction from which the wind blows. In open water, if the wind continues to blow, as happens in the Roaring Forties in the southern hemisphere, long, organised masses of water called swell roll across the ocean. If the wind dies down, the wave formation is reduced but waves already formed continue to travel in their original direction until they meet land. Small waves form in small areas of water with islands and other land masses but large waves form in open stretches of sea where the wind blows steadily and strongly. When waves meet other waves coming from different directions, interference between the two can produce broken, irregular seas.
The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the sea by the wind but this represents a transfer of energy and not a horizontal movement of water. When a wave approaches, the water molecules at a point rise up and when it retreats, they go down, moving in a roughly circular pattern each time a wave passes. Those near the surface make a larger movement than those lower down, and deep molecules are completely unaffected by the passage of a wave. A floating object rises up and down as a wave passes but is not moved along by the wave (only by the wind). When waves approach land and move into shallow water, they change their behaviour. If approaching at an angle, the waves may bend or wrap around objects such as rocks or headlands. When the deepest circling molecules in a wave come into contact with the seabed, friction between the water and the beach slow the wave down and the crests become closer together. The height of the wave increases as the energy in it is unable to move downwards and is forced upwards instead. The wave changes profile as the crest moves faster than the base. Eventually, the wave "breaks" as it topples forward and is converted into a tumbling mass of foamy water. This rushes in a sheet up the beach before retreating back into the sea under the influence of gravity.
A tsunami is a very unusual form of wave and is caused by some geological event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. This trigger event temporarily lifts the surface of the water, usually by a few feet (one meter). The potential energy of the raised sea above the site is turned to kinetic energy creating a shallow wave, known as a tsunami, radiating outwards at a speed proportional to the square of the sea depth. A trigger event on the continental shelf may cause a local tsunami on the land side and a distant tsunami that travels out across the ocean. Normal surface waves are up to 45 feet (14 metres) high, have a wavelength of a few hundred ft (one hundred meters) and travel at up to 65 miles per hour (105 km/h). Tsunami have a wavelength of 80 to 300 miles (130 to 480 km) and travel about ten times faster. In the open sea they may pass unnoticed as their height at this stage is usually less than three feet, it is as they enter shallower water that their dimensions change.
As a tsunami approaches the coast and the water becomes shallower, the wave is compressed and its speed decreases to below 80 miles per hour (130 km/h). Its wavelength diminishes to less than twelve miles and its amplitude increases enormously. Its behaviour is similar to a wind-generated wave but the scale is vastly different and involves not just the surface layers of the sea but the whole water column. The water in front of the wave may get sucked back and added into the crest leaving the seabed close to the shore exposed. The wave begins to tower up in the same way as a normal wave but on a vastly greater scale. It does not usually break but it rushes inland, engulfing all in its path. Much of the destruction wreaked may be caused by the water draining back into the sea after the wave has struck, dragging debris and people with it. Often several tsunami are caused by the single geological event and arrive at intervals of somewhere between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive.
Wind blowing over the surface of the sea causes friction at the interface between air and sea. Not only does this cause waves to form but it also makes the surface of the water move. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics. When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as a gyre is formed. There are five main gyres in the world's oceans, two in the Pacific, two in the Atlantic and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows round Antarctica. These gyres have followed the same routes, guided by the topography of the land, the wind direction and the Coriolis effect for millennia. The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes.
Surface currents only affect the top few hundred metres (yards) of the sea but there are also large scale flows in the ocean depths caused by the movement of deep water masses. There is a main deep ocean current which flows through all the world's oceans and is known as the thermohaline circulation or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes more salty as sea ice crystallizes out. Both these factors make it denser and the water sinks. From the deep sea near Greenland, such water flows southwards between the continental land masses on either side of the Atlantic. When it reaches the Antarctic it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams which move northwards into the Indian and Pacific Oceans. Here it is gradually warmed and becomes less dense, rises towards the surface and loops back on itself. Some flows back into the Atlantic. It takes a thousand years for this circulation pattern to be completed.
Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, can create spits, make beaches disappear and water channels silt up. A rip current can occur when water piles up near the shore from advancing waves and is funnelled out to sea through a channel in the seabed. It may occur at a gap in a sandbar or near a man-made structure such as a groin. These strong currents can have a velocity of 3 ft (0.9 m) per second, can form at different places at different stages of the tide and can carry away unwary bathers. Another temporary current is an upwelling current which occurs when the wind pushes water away from the land and deeper water rises to replace it. This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea.
Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational pull of the Moon and the Sun and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore which is known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range.
Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minutes that the Moon takes to make a complete rotation of the Earth and return to the same position in the sky. The Moon is 27 million times smaller than the Sun, but it is 400 times closer to the Earth. This means that its gravitational pull at the Earth's surface is more than twice as great as that of the Sun. The tidal force caused by the Moon draws the sea towards it while inertial forces act to keep the water in place. The gravitational force is stronger so a bulge forms in the ocean at the place where the Earth and Moon are at their closest. On the opposite side of the globe, the lunar force is at its weakest and the inertial force is stronger than the gravitational pull and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the globe. The gravitational force of the Sun is also working on the seas. It is less powerful than that of the Moon and when the Sun, Moon and Earth are all aligned (full moon and new moon), their combined gravitational pulls results in abnormally high "spring tides". When the Sun is at 90° from the Moon as viewed from Earth, it counteracts the pull of the Moon and the tidal range is smaller, causing "neap tides" to occur.
In places like the Gulf of Mexico where land constrains the movement of the bulges, only one set of tides may occur each day. Local topography also has an effect. Inshore from an island there may be a complex four high tide diurnal cycle. Where there is a funnel-shaped bay or estuary the tidal range can be magnified. The Bay of Fundy is the classic example of this and can experience spring tides of 15 m (49 ft). Although tides are regular and predictable, the height of high tides can be lowered by offshore winds and raised by onshore winds. The high pressure at the centre of an anticyclones pushes down on the water and is associated with abnormally low tides while low-pressure areas may cause extremely high tides. A storm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low pressure system, can raise the surface of the sea at high tide dramatically. In 1900, Galveston, Texas experienced a 15 ft (5 m) surge during a hurricane which overwhelmed the city, killing over 3,500 people and destroying 3,636 homes.
Life in the sea 
A large proportion of all life on Earth exists in the oceans, which provide about 300 times as much habitable volume as terrestrial habitats. Marine habitats range from surface water to the deepest oceanic trenches, including coral reefs, kelp forests, seagrass meadows, tidepools, muddy, sandy and rocky bottoms, and the open pelagic zone. The organisms living in the sea range from microscopic phytoplankton and zooplankton to whales up to 30 metres (100 ft) long. Marine life is economically important to humans, especially the fish used for food, and provides support for the carbon cycle.
Life probably originated in the sea, given that all living things are made mainly of water, and all the major groups of animals are represented in the sea. Scientists differ on which part of the sea gave rise to life: the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea "black smoker" vents, all of which would have provided protection from damaging ultraviolet radiation, not blocked by the early earth's atmosphere.
Marine habitats 
Marine habitats can be divided (horizontally) into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf. Alternatively, marine habitats can be divided (vertically) into pelagic (open water) and demersal (sea bottom) habitats. A third division is by latitude: from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters.
Coral reefs, the so-called "rainforests of the sea", occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are tropical coral reefs such as Australia's Great Barrier Reef, but cold water reefs harbour a wide array of species including corals (only six of which contribute to reef formation).
Primary producers 
Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very diverse. Microscopic photosynthetic algae, phytoplankton, contribute a larger proportion of the world's photosynthetic output than all the terrestrial forests combined. About 45 percent of the sea's primary production of living material is contributed by diatoms. Much larger algae, commonly known as seaweeds, are important locally; Sargassum forms drifts, while kelp form kelp forests. Flowering plants in the form of seagrasses grow in "meadows" in sandy shallows. Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow. Productivity is not necessarily higher in warmer waters; it is instead controlled mainly by upwelling of cold but mineral-rich waters and near rivers which bring nutrients leached from the soil, so the most productive zones, rich in plankton and so also in fish, are mainly coastal.
Marine invertebrates make up a large part of all life in the sea. The major groups (phyla) include Cnidaria such as jellyfish and sea anemones; Ctenophora, the comb jellies; sea worms of many phyla including Platyhelminthes (flatworms), Nemertea, Annelida (true or segmented worms), Sipuncula, Echiura, Chaetognatha, and Phoronida; Mollusca including Gastropods (sea snails), Bivalves such as clams, and Cephalopods (octopuses, squids); Arthropoda including Chelicerata and Crustacea such as prawns; Porifera, the sponges; Bryozoa, small colonial coral-like animals; Echinodermata including starfish and sea urchins; and Urochordata including sea squirts or tunicates. Marine vertebrates include fish, mammals such as dolphins, and birds such as gulls and albatrosses.
Humans and the sea 
Humans have navigated the seas since antiquity. The Ancient Egyptians and Phoenicians navigated the Mediterranean Sea and the Red Sea, while the Egyptian Hannu sailed along the Red Sea, reaching the Arabian Peninsula and the African Coast around 2750 BC. In the 1st millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the Black Sea. The seas along the eastern and southern Asian coast were used by the Arabs, Chinese and Pacific islanders for navigation. The Polynesians were experts at navigation, passing their knowledge on verbally from generation to generation. In their timber boats lashed together with braided fibres they travelled thousands of miles between tiny islands using the stars, the direction of swells and other simple signs to find their way.
In the Dark Ages, the Vikings in their longships navigated the North Atlantic from Scandinavia as far north and west as Iceland, Greenland, and Newfoundland, and as far south as the Mediterranean; it is possible they reached the Indian Ocean, rounding the Cape of Good Hope, but made little contact with the Arab navigators.
In the mediaeval to modern period, Western European mariners made voyages of exploration in search of trade starting in the fifteenth century. Bartolomeu Dias rounded the Cape of Good Hope in 1487. Vasco de Gama reached India via the Cape in 1498. Christopher Columbus sailed from Cadiz in 1492 to reach the Eastern lands (of India or Japan) by going westwards around the world, making landfall instead on an island in the Caribbean Sea. The English navigator John Cabot reached Newfoundland in 1497, again hoping to reach the profitable East. The Italian Amerigo Vespucci reached South America, sailing south to the mouth of the River Amazon; the Americas were named after him.
The Portuguese navigator Ferdinand Magellan led the first expedition to sail around the world. He started in 1519, reaching the Straits of Magellan in 1520 having lost two of his five ships, and crossing the Pacific, nearly starving, reaching Guam in March 1521. Magellan was killed soon afterwards; only one of his ships, under Sebastian del Cano, returned safely to Seville in September 1522.
By 1700, the seas along the Arctic and Pacific coasts of Russia had been discovered, but accurate charting only began in the 18th century, and Severnaya Zemlya was not discovered until 1910. The Dutch captain Willem Barents sailed beyond Norway to Svalbard and the Barents Sea. Gerardus Mercator made a practical map of the world, using the projection named after him which conveniently makes navigation rhumb lines straight. However, effective navigation opened the seas to piracy and war. Skilled mariners such as the Englishman Francis Drake and the Spanish conquistadors were rewarded by their home countries for successful and often violent exploitation. Far from discovering empty lands, they subjugated and looted the peoples whose lands they found.
By no means all mediaeval navigators were from Western Europe. The Novgorodians have sailed the White Sea since at least the 13th century. The Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in the early fifteenth century, sailing the Indian and Pacific Oceans.
Origins of oceanography 
Scientific oceanography can be considered to have begun with the three voyages of Captain James Cook from 1768 to 1779, exploring, charting and describing the South Pacific with unprecedented precision from 71 degrees South to 71 degrees North. John Harrison's chronometers supported Cook's accurate navigation and charting on two of these voyages, permanently improving the standard attainable for subsequent work. Other expeditions followed in the nineteenth century, from Russia, France, the Netherlands and the United States as well as Britain. On HMS Beagle, which provided Charles Darwin with ideas and materials for his 1859 book On the Origin of Species, the ship's captain, Robert FitzRoy, charted the seas and coasts and published his four volume report of the ship's three voyages in 1839. Edward Forbes's 1854 book, Distribution of Marine Life, had a wide influence on research on the world's seas, though he argued that no life could exist below around 600 metres (2000 feet). In this, Forbes was proven wrong by the British biologists W. B. Carpenter and C. Wyville Thomson, who in 1868 discovered life in deep water by dredging. Wyville Thompson became chief scientist on the Challenger expedition of 1872–1876, which effectively created the science of oceanography.
People have made use of the sea for millennia, for trade, warfare, travel and fishing, and more recently for leisure, power generation, and extraction of minerals. Many of these activities have caused marine pollution.
Large quantities of commodities and merchandise are transported by sea, especially across the Atlantic and around the Pacific rim. A major trade route passes through the Pillars of Hercules, across the Mediterranean and the Suez Canal to the Indian Ocean and through the Straits of Malacca; much trade also passes through the English Channel. Shipping lanes are the routes on the open sea used by cargo vessels. In the days of sail, these were influenced by the trade winds and currents and there are still advantages of using these routes today as they allow vessels to maintain an even keel. Over sixty percent of the world's container traffic is conveyed on the top twenty trade routes. Shipping is supplemented by air freight in which cargoes are moved by aircraft but this is a more expensive process and is mostly used for particularly valuable or perishable cargoes. Seaborne trade carries more than US $4 trillion worth of goods each year.
There are two main kinds of freight, bulk cargo and break bulk or general cargo. Commodities are bulky goods in the form of liquids, powder or particles and include oil, grain, coal, ore, scrap metal, sand and gravel. These cargoes are carried loose in the holds of bulk carriers. Break bulk cargo is usually manufactured goods and is transported in packages, often stacked on pallets. Before the arrival of containerization in the 1950s, these goods were loaded into the hold, stacked and secured, transported to their destination, unstacked and unloaded in a piecemeal fashion. The increased use of containers has greatly increased the efficiency and decreased the cost of moving these goods to international destinations. The use of containers also improves the efficiency of moving the goods from the warehouse to the ship and from the port of arrival to the final destination.
Most freight now travels in containers that are loaded on board purpose-built container ships at dedicated container terminals. The hold of the ship is specially designed to speed loading and unloading and keep containers secure while at sea. Features include hatches extending across the whole width of the vessel and the use of cell guides to facilitate neat, efficient stacking. Other containers are carried on deck. The containers are lockable steel boxes in standard sizes; this facilitates handling and stacking, and prevents pilfering. After delivery to their destinations, the empty containers are reused. Freight forwarding firms will book the cargo, arrange pickup and delivery times and manage the necessary documentation.
Food production 
Coastal countries have an exclusive economic zone extending 200 miles (320 km) from their shores in which they have rights to the marine resources including the fish stocks and may regulate the species, size and total weight of catch permitted. About 87 percent of fish are caught in these zones but fishing vessels are increasingly venturing further afield to exploit stocks in international waters. In 2011, total world production of fish, including aquaculture, was estimated to be 154 million tonnes of which 131 million tonnes was for human consumption. Direct caught fish accounted for 90 million tonnes while aquacultural production has been increasing annually and now contributes about one third of the total world production of fish products. The north west Pacific is the most productive area while fish catches in most of the other ocean areas peaked several years ago. Fish stocks are being fully exploited in some areas while in others, more fish are being removed than can be replenished by natural means. The number of vessels employed in sea fishing is over 3 million.
Modern fishing vessels include fishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels and large factory ships which are designed to stay at sea for weeks, catching, processing and freezing great quantities of fish. The equipment used to capture the fish may be purse seines, other seines, trawls, dredges, gillnets and long-lines and the fish species most frequently targeted are herring, cod, anchovy, tuna, flounder, mullet, squid and salmon. Fishing methods vary between species and many countries have introduced quotas in their own waters.
At the other extreme, artisan fishing methods include low-technology systems including rod and line, harpoons, skin diving, traps, throw nets and drag nets. Traditional fishing boats are employed, powered by paddle, wind or outboard motors and operating in near-shore waters. The FAO is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.
As well as the wild stock, about 79 million tonnes of food and non-food products were produced by sea farming in 2010, an all time high. About six hundred species of plants and animals were cultured in this way, some for use in seeding wild populations. The animals raised included finfish, aquatic reptiles, crustaceans, molluscs, sea cucumbers, sea urchins, sea squirts and jellyfish. Integrated mariculture has an advantage over more traditional aquacultural practice in that there is a readily available supply of planktonic food and waste is removed naturally. Various methods are employed. Mesh enclosures for finfish can be suspended in the open seas, cages can be used in more sheltered waters or ponds can be prepared which are frequently refreshed with water at each high tide. Shrimps can similarly be reared in shallow ponds connected to the open sea. Ropes can be hung in water for the cultivation of algae, oysters and mussels. Oysters can be reared on trays or in mesh tubes. Sea cucumbers can be ranched on the seabed. Captive breeding programmes can be developed and juveniles can be liberated into the wild. Lobster larvae have been successfully raised in this way in Maine resulting in an increase in the numbers of lobsters harvested locally. 145 species of seaweed – red, green, and brown algae – are eaten worldwide, and some have long been farmed in Japan and other Asian countries; there is great potential for additional food production. There are in contrast relatively few maritime flowering plants that are widely used for food; one is marsh samphire, which is eaten both raw and cooked.
Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century. Maritime leisure activities are varied, including self-organized trips cruising, sailing, fishing, commercially organized voyages on cruise ships, and trips on smaller vessels for ecotourism such as whale watching and coastal birdwatching.
Although not at home in the water in the same way as whales, seals or penguins, humans still enjoy venturing into the sea. Children paddle and splash in the shallows and many people enjoy swimming and surfing. Beneath the surface, spearfishing and freediving are necessarily restricted to surface waters. Pearl divers have traditionally greased their skins, put cotton in their ears and clips on their noses and dived to 40 feet (12 m) with baskets to collect oysters. Human eyes are not adapted for use underwater but vision can be improved by wearing a diving mask. The possibilities for exploration of the submarine environment are further extended by the use of flippers and snorkels, and scuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface. Diving suits can be worn and buoyancy can be adjusted through the use of weights.
The depths which can be reached by divers and the length of time they can stay underwater is limited by the increase of pressure they experience as they descend and the need to prevent decompression sickness as they return to the surface. Recreational diving is limited to depths of 100 feet (30 m) beyond which nitrogen narcosis may occur. Deeper dives can be made with specialised equipment and training. A new world record was set in 2006 when a US Navy diver descended to 2,000 feet (610 m) using a pressurized atmospheric diving suit. Beyond this depth, it is necessary to use specialist vehicles, usually tethered to a parent ship, either remotely operated underwater vehicles with lights and cameras or manned submersibles. At great depths, no light penetrates through the water layers from above and the alien presence of the submersible and its lighting equipment has unavoidable consequences for the marine organisms nearby which may behave in an abnormal manner. When these deep sea creatures are brought to the surface they usually die because of the changes in pressure that they experience. Modern equipment with LED lights and low light cameras has helped researchers to study their behaviours and characteristics in their natural habitat. However, deep sea research is very much in its infancy. 
Power generation 
The sea offers a very large supply of kinetic energy which is carried by ocean waves, tides, salinity differences, and ocean temperature differences. This renewable energy, derived ultimately from the flow of energy from the sun, can be harnessed to generate electricity. Forms of renewable marine energy include tidal power, marine current power, osmotic power, ocean thermal energy, and wave power.
Tidal power requires a dam to store and then release seawater, making it an option in parts of the world such as Brittany where the tidal range is large. The Rance barrage, 1 km long, near St Malo opened in 1967; it generates about 0.5 GW, but it has been followed by few similar schemes. A 7 GW scheme for the Severn Estuary, which has the world's second-largest tidal range, has been proposed.
The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop. A small 2 MW commercial wave power plant, "Osprey", was built in Northern Scotland in 1995 about 300 metres offshore. It was quickly damaged by waves, then destroyed by a storm.
Marine current power is a relatively predictable energy source, since unlike wind or solar power it is available around the clock, and it could provide populated areas close to the sea, such as Florida, with a significant part of their energy needs. In principle, marine current power could be harnessed by a variety of types of open-flow turbine, which could be supported by systems on the sea bed, floating and moored, or intermediate; sea bed systems are already available, but are limited to a depth of about 40 metres.
Osmotic or salinity gradient power exploits the energy available across a membrane with salt water one side and fresh water on the other. Power could be obtained by either of two technologies that have been tested in the laboratory, reverse electrodialysis and pressure-retarded osmosis.
Ocean thermal energy conversion exploits the small temperature difference between cooler deep and warmer shallow or surface waters to produce electricity with a heat engine using the Rankine cycle. The technology can employ either an open or a closed cycle. The closed cycle uses an evaporator, a turbogenerator, and a condenser to reuse a working fluid, like a refrigerator working in reverse, and depends critically on costly heat exchangers. The open cycle avoids heat exchangers but is vulnerable to dissolved gases, and works at low pressure so it requires very large flow rates and hence large turbines. Despite the promise of thermal energy conversion, the projected cost per kiloWatt has prevented its use for commercial energy production, though small-scale uses for cooling and mariculture are in operation.
Offshore wind power is (in 2011) a more mature technology than marine energy. It is captured by wind turbines placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore. Offshore wind power capacity grew rapidly between 2000 and 2010. The first offshore wind farm was installed in Denmark in 1991. The installed capacity of offshore wind farms in European waters reached 3 GW in 2010. Wind power provides clean, renewable electric power, but to capture a significant share of the energy market it needs to improve its performance, reliability and cost from 2011 levels. There are other challenges: large wind farms affect local meteorology; there can be impacts on wildlife such as birds; wind power is variable and unpredictable, and like solar power it cannot be "dispatched" to provide energy to meet cycles of consumer demand.
Extractive industries 
The seabed contains enormous reserves of minerals. Exploitation of these in shallow water on continental shelves is done by dredging. This process has certain advantages over land-based mining in that the construction of equipment can be done at specialised shipyards and the infrastructure costs are lower. Disadvantages include the problems caused by the waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps. There is a need to consider possible coastal erosion and the damage done to the environment.
Seafloor massive sulphide deposits have been recognised as potential sources of silver, gold, copper, lead and zinc as well as other trace metals since their discovery in the 1960s. They are the result of deposits formed when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The grades of ore are high but the cost of extraction is currently prohibitive. Small scale mining of the deep sea floor is being developed off the coast of Papua New Guinea using modern technological advances and robotic techniques but the obstacles to creating a viable undersea commercial enterprise are formidable. Environmentalists are concerned that large-scale damage might be done by mining operations and that animal communities clustered round underwater vents might be destroyed.
There are large deposits of petroleum in the form of oil and natural gas in rock formations beneath the seabed. Offshore platforms and drilling rigs are used to extract the oil or gas and store it until it can be transported to land. Offshore oil and gas production is more challenging than on-land due to the remote, harsh environment. There is a trend towards conducting more of the production operations on the seabed, by separating water from oil there or by pumping it onshore with no installations visible above the sea. Mobile offshore drilling units are used to drill the initial well and are then moved elsewhere and by this means, deeper water deposits can be exploited. Drilling for oil in the sea has environmental impacts. Animals may be disorientated by seismic waves used to locate deposits and these are believed to have caused the beaching of whales. Toxic substances such as mercury, lead and arsenic may be released into the sea. The infrastructure may cause damage and the oil may be spilt. As a result of the explosion that destroyed the Deepwater Horizon drilling rig in the Gulf of Mexico in 2010, new safety standards have been introduced into the industry and advances have been made in controlling released oil.
Large quantities of methane clathrate exist on the seabed and in ocean sediment at a temperature of around 2 °C (36 °F) and these are of interest as a potential energy source. Some estimates of the potential resource put the amount available at between one and five million cubic kilometres (0.24 to 1.2 million cubic miles). Also lying on the seabed are manganese nodules formed of concentric layers of iron, manganese and other hydroxides around a core. In the Pacific these are particularly numerous and may cover ten to thirty percent of the deep ocean floor. Some are shallowly buried in the sediment but few are found underneath it. The minerals precipitate from seawater and grow very slowly. Their commercial extraction was investigated in the 1970s, primarily for the nickel they contained, but was abandoned when more convenient sources of the metal were discovered.
The sea holds enormous quantities of valuable minerals dissolved in the water. Salt for table and industrial use is the most important of these and has been harvested by solar evaporation from shallow ponds since prehistoric times. Another element obtained from the sea is bromine which has accumulated there after leaching from the land. It is economically recovered from the Dead Sea where it occurs at 55,000 ppm. Magnesium can be obtained by adding lime to seawater and precipitating the resulting insoluble hydroxide. Gold is present in seawater in concentrations of 0.000011 ppm and uranium 0.0033 ppm. The German chemist Fritz Haber tried extracting the gold in the 1920s but completely failed. Periodically, attempts to recover uranium are made, especially at times of world energy shortage. The accepted method of pumping water through a membrane uses so much energy that it is not a practical strategy for uranium extraction. An alternative mechanism would be dropping a membrane into the sea in an area in which strong currents cause water to flow past the adsorbing surface. However, the total expenditure of energy relative to the amount of uranium that could be recovered by these means, makes extracting the element from the sea not a practical possibility.
Desalination is the technique of removing salts from seawater to leave fresh water suitable for drinking or irrigation. The two main processing methods, vacuum distillation and reverse osmosis, use large quantities of energy. Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations. The brine produced as a by-product contains some toxic materials and is returned to the sea. An alternative method of producing pure water is as part of an Integrated biotectural system which relies on solar thermal energy or wind power for distillation and produces sea salt as a by-product. This type of process is being used on a limited scale in the Middle East and North Africa.
Control of the sea is important to the security of a maritime nation and the naval blockade of a port can be used to cut off food and supplies in time of war. Battles have been fought on the sea for more than 3,000 years. In about 1210 B.C., the king of the Hittites defeated and burned a fleet from Cyprus. In the decisive 480 B.C. Battle of Salamis, the Greek general Themistocles trapped the far larger fleet of the Persian king Xerxes in a narrow channel and attacked vigorously, destroying 200 Persian ships for the loss of 40 Greek vessels. At the end of the Age of sail, the English navy led by Horatio Nelson broke the power of the combined French and Spanish fleets at the 1805 Battle of Trafalgar, though Nelson himself was killed in the battle.
With steam and the industrial production of steel plate came greatly increased firepower in the shape of the Dreadnought battleships armed with long range guns. These fought inconclusively in the First World War at the 1916 Battle of Jutland between the Royal Navy's Grand Fleet and the Imperial German Navy's High Seas Fleet. In the Second World War, the British victory at the 1940 Battle of Taranto showed that naval air power was sufficient to overcome the largest warships, foreshadowing the decisive sea-battles of the Pacific War including the Battle of the Coral Sea, the Battle of Midway, the Battle of the Philippine Sea, and the climactic Battle of Leyte Gulf, in all of which the dominant ships were Aircraft carriers.
Marine pollution 
Plastic drink bottles, polythene bags, cigarette lighters, tubs and containers get into rivers and are carried out to sea where all this plastic trash does not biodegrade. Instead it disintegrates over time, breaking into smaller and smaller pieces, eventually down to the molecular level. Rigid plastics tend to remain undegraded. The Great Pacific Garbage Patch is a name given to a floating accumulation of mostly plastic waste in the middle of the Pacific gyre. This junk from Japan and the west coast of the United States is still floating here ten and even twenty years after it was discarded. More is added to the trash pile every year. Foraging sea birds such as the albatross, fulmar, shearwater and petrel may mistake the debris for food and the indigestible plastic accumulates inside them. Turtles and whales do likewise and have been found with plastic bags, fishing line and other plastics in their stomachs. Many plastic items float but tiny fragments, known as microplastics, may sink and pose threats to filter feeders and other marine invertebrates at the bottom of the food chain.
Oil spills do much damage to the marine environment, but the majority of oil found in the sea comes from waste and run-off from cities and industry. Oil can have a devastating effect on marine animals. It can clog the feathers of sea birds, reducing their insulating effect and the birds' buoyancy, and be ingested when they preen themselves in an attempt to remove the contaminant. Marine mammals are less seriously affected but may be chilled through the removal of their insulation, blinded, dehydrated or poisoned. Benthic invertebrates are swamped when the oil sinks, fish are poisoned and the food chain is disrupted. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected and the livelihoods of people dependant on the sea being devastated. Fortunately, the marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove the oil from the sea.
Run-off of fertilisers from agricultural land is a great source of pollution in some areas and the discharge of raw sewage has a similar effect. The extra nutrients provided by these sources can cause eutrophication. Nitrogen is often the limiting factor in marine systems and with added nitrogen, algal blooms and red tides can occur which lower the oxygen level of the water and kill marine animals. This pollution has caused dead zones in some parts of the world such as the Baltic Sea and Gulf of Mexico. Some algal blooms are caused by cyanobacteria that contain toxins that build up in the tissues of shellfish that feed on them. The molluscs are apparently unharmed, but other wildlife such as sea otters that eat them can be severely affected.
Many substances enter the sea through various routes as a result of human activities. Combustion products from vehicles, factories and homes are transported in the air and deposited into the sea by precipitation. Industrial outflows and sewage treatment plants contribute heavy metals and other toxic substances to waterways and seas. Pesticides, disinfectants, household cleaning products and other synthetic chemicals follow the same route and end up in the sea. Although these products may be present in minute proportions in the open ocean, there is a higher concentration of them in the surface film and in the marine sediment, especially in estuarine mud. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects. However, developmental problems, behavioural abnormalities, cancer and other diseases have been linked to this contamination with fish, birds and mammals being affected. The heavy metals of greatest concern are copper, lead, mercury, cadmium and zinc. These and other substances are bio-accumulated by marine invertebrates which have no metabolic pathways to deal with them. They get passed up the food chain and are cumulative toxins. PCBs and heavy metals have been implicated in the diseased state of the bottlenose dolphin populations inhabiting the east coast of Florida. This ill health may be caused by the suppression of their immune systems by these contaminants leaving them open to opportunistic infections.
In culture 
People experience the sea in contradictory ways: as powerful but serene, beautiful but dangerous. Human reactions to the sea are found in, for example, literature, art, poetry, film, theatre, and classical music, as well as in mythology and the psychotherapeutic interpretation of dreams.
In literature 
The sea has appeared in literature since at least the time of the Ancient Greek poet Homer's epic poem The Odyssey, written in the 8th century BC. The poem describes the voyage of ten years of the Greek hero Odysseus as he struggles to return home across the sea after the war with Troy described in the Iliad. His wandering voyage takes him from one strange and dangerous land to another, experiencing among other maritime hazards shipwreck, the sea-monster Scylla, the whirlpool Charybdis and the delightful island Calypso.
The soldier Xenophon, in his Anabasis, told how he witnessed the roaming 10,000 Greeks seeing the Black Sea from Mount Theches, after participating in Cyrus the Younger's failed march against the Persian Empire in 401 BC. The 10,000 joyfully shouted "Thálatta! Thálatta! "(Greek: Θάλαττα! θάλαττα!) — "The Sea! The Sea!"
In modern European literature, the novelist Joseph Conrad stands out, with sea-inspired books including An Outcast of the Islands (1896), The Nigger of the 'Narcissus' (1897), Lord Jim (1900), Typhoon (1902) and The Shadow Line (1916), all of which draw on his personal experience of the sea: he had been a Captain in the merchant navy:
The Nan-Shan was ploughing a vanishing furrow upon the circle of the sea that had the surface and the shimmer of an undulating piece of gray silk.—Typhoon, by Joseph Conrad
Herman Wouk writes that "Nobody, but nobody, could write about storms at sea like Conrad." Wouk cites Conrad's narrator in The Nigger of the 'Narcissus': "We had so far saved him ["Jimmy, the black crewman"]; and it had become a personal matter between us and the sea."
In American literature, perhaps the best-known maritime work is Herman Melville's 1851 novel Moby Dick, describing the adventures of the sailor Ishmael, the whaleship Pequod, its Captain Ahab, and the white sperm whale, Moby Dick. It is possible to take an academic course in the literature of the sea.
In art 
People have depicted ships and boats in art for centuries, but the genre of marine art became especially important in the paintings of the Dutch Golden Age, with works showing the Dutch navy at the peak of its military prowess. Artists such as Jan Porcellis, Simon de Vlieger, Jan van de Cappelle, Hendrick Dubbels, Willem van de Velde the Elder and his son, Ludolf Bakhuizen and Reinier Nooms created maritime paintings in a wide variety of styles.
The Japanese artist Katsushika Hokusai created colour prints of the moods of the sea, including "The Great Wave Off the Coast of Kanagawa" showing the destructive force of the sea at the same time as its ever-changing beauty.
In poetry 
In poetry, the sea offers a variety of possibilities.
- araumi ya / Sado ni yokotau / amanogawa
- the rough sea / stretching out towards Sado / the Milky Way 
The albatross played a central part in Samuel Taylor Coleridge's influential 1798 poem The Rime of the Ancient Mariner, which in turn gave rise to the usage of albatross as metaphor for a burden.
- Who hath desired the Sea?—the sight of salt water unbounded—
- The heave and the halt and the hurl and the crash of the comber wind-hounded?
while the poem's refrain runs:
- His Sea in no showing the same—his Sea and the same 'neath each showing : His Sea as she slackens or thrills ?
Even in humorous verse for children, the sea plays a role, as in Edward Lear's 1871 poem The Owl and the Pussycat, which begins with the lines "The Owl and the Pussycat went to sea | In a beautiful pea-green boat".
In cinema 
In cinema, the sea has played roles from central protagonist to villain from the earliest days of cinema, starting with Louis Lumière's silent 1895 documentary La Mer. The 1953 film The Cruel Sea begins with a voice-over by the actor Jack Hawkins including the words "The only villain is the sea, the cruel sea, that man has made more cruel..." It was based on Nicholas Monsarrat's 1951 novel The Cruel Sea. The novelist Herman Wouk, reviewing "five best nautical yarns", writes that "'The Cruel Sea' was a major best seller and became a hit movie starring Jack Hawkins. Its authenticity is unmistakable... His description of a torpedoed crew, terrified, clinging to life rafts in the blackest of nights, is, indeed, too authoritative for comfort—we soon feel ourselves in that sea." Films such as Wolfgang Petersen's The Perfect Storm (2000) play on the violence of the sea.
In theatre 
In theatre, William Shakespeare's 1610–11 play The Tempest has a stormy sea as a major element. The Norwegian playwright Henrik Ibsen's 1888 play The Lady from the Sea (Norwegian: Fruen fra havet) tells the story of the daughter of a lighthouse keeper who grew up beside the sea. She marries a small-town doctor but remains emotionally attached to a seaman. Eventually she has to decide between the two.
In classical music 
The sea has inspired much music over the centuries. Richard Wagner claimed that his 1843 opera The Flying Dutchman was inspired by a memorable sea crossing from Riga to London, his ship being delayed in the Norwegian fjords at Tvedestrand for two weeks by storms. The French composer Debussy's 1903–05 work La mer, trois esquisses symphoniques pour orchestre (the sea, three symphonic sketches for orchestra), completed on the English Channel coast, evokes the sea with "a multitude of water figurations". Other works composed at about this time include Stanford's Songs of the Sea (1904) and Songs of the Fleet (1910), Elgar's Sea Pictures (1899) and Vaughan Williams' choral work, A Sea Symphony (1903–1909). The English composer Frank Bridge, born in the coastal town of Brighton, wrote an orchestral suite called The Sea in 1911. Four Sea Interludes (1945) is an orchestral suite by Benjamin Britten that forms part of his opera, Peter Grimes.
In symbolism: mythology and psychotherapy 
Symbolically, the sea has long been perceived as a hostile and dangerous environment populated by fantastic creatures: the Leviathan of the Bible, Scylla in Greek mythology, Isonade in Japanese mythology, the Kraken of late Norse mythology).
- [Dream] By the sea shore. The sea breaks into the land, flooding everything. Then the dreamer is sitting on a lonely island.
- [Interpretation] The sea is the symbol of the Collective unconscious, because unfathomed depths lie concealed beneath its reflecting surface.
In a footnote to this, Jung writes "The sea is a favourite place for the birth of visions (i.e. invasions by unconscious contents)".
List of seas 
See also 
- International Maritime Organization
- List of places on land with elevations below sea level
- Pole of inaccessibility: the locations farthest from any coastline
- Sea level
- Sea level rise
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- often treated as part of Mediterranean Sea
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