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Cave diving is underwater diving in water-filled caves. It may be done as an extreme sport, a way of exploring flooded caves for scientific investigation, or for the search for and recovery of divers lost while diving for one of these reasons. The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave diving is done using scuba equipment, often in specialised configurations. Recreational cave diving is generally considered to be a type of technical diving due to the lack of a free surface during large parts of the dive, and often involves decompression.
In the United Kingdom, cave diving developed from the locally more common activity of caving. Its origins in the United States are more closely associated to recreational scuba diving. Compared to caving and scuba diving, there are relatively few practitioners of cave diving. This is due in part to the specialized equipment and skill sets required, and in part because of the high potential risks due to the specific environment.
Despite these risks, water-filled caves attract scuba divers, cavers, and speleologists due to their often unexplored nature, and present divers with a technical diving challenge. Underwater caves have a wide range of physical features, and can contain fauna not found elsewhere.
- 1 Procedures
- 2 Hazards
- 3 Safety
- 4 Equipment
- 5 International differences
- 6 History
- 7 Cave diving venues
- 7.1 Africa
- 7.2 Asia
- 7.3 Australia
- 7.4 Europe
- 7.5 North America
- 7.6 South America
- 8 See also
- 9 References
- 10 Sources
- 11 External links
The procedures of cave diving have much in common with procedures used for other types of penetration diving. They differ from open water procedures mainly in the emphasis on navigation, gas management, operating in confined spaces, and that the diver is physically constrained from direct ascent to the surface during much of the dive.
As most cave diving is done in an environment where there is no free surface with breathable air allowing an above-water exit, it is critically important to be able to find the way out before the breathing gas runs out. This is ensured by the use of a continuous guideline between the dive team and outside of the flooded cave, and diligent planning and monitoring of gas supplies. Two basic types of guideline are used: permanent lines, and temporary lines. Permanent lines may include a main line starting near the entrance/exit, and side lines or branch lines, and are marked to indicate the direction to the nearest exit. Temporary lines include exploration lines and jump lines.
Decompression procedures may take into account that the cave diver usually follows a very rigidly constrained and precisely defined route, both into and out of the cave, and can reasonably expect to find any equipment such as drop cylinders temporarily stored along the guideline while making the exit. In some caves, changes of depth of the cave along the dive route will constrain decompression depths, and gas mixtures and decompression schedules can be tailored to take this into account.
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Most open-water diving skills apply to cave diving, and there are additional skills specific to the environment, and to the chosen equipment configuration.
- Good buoyancy control, trim and finning technique help preserve visibility in areas with silt deposits.
- The ability to navigate in total darkness using the guideline to find the way out is a safety critical emergency skill. Line management skills required for cave diving include laying and recovering guide lines using a reel, tie-offs, the use of a jump line to cross gaps or find a lost guide line in silted out conditions, identifying the direction along the guideline leading to the exit, and the skills of dealing with a break in a guideline.
- Emergency skills for dealing with gas supply problems are complicated by the possibility of the emergency occurring in a confined space and low visibility or darkness.
Cave diving training includes equipment selection and configuration, guideline protocols and techniques, gas management protocols, communication techniques, propulsion techniques, emergency management protocols, and psychological education.[clarification needed] Cave diver training also stresses the importance of risk management and cave conservation ethics. Most training systems offer progressive stages of education and certification.
- Cavern training covers the basic skills needed to enter the overhead environment. Training will generally consist of gas planning, propulsion techniques needed to deal with the silty environments in many caves, reel and handling, and communication. Once certified as a cavern diver, a diver may undertake cavern diving with a cavern or cave certified "buddy", as well as continue into cave diving training.
- Introduction into cave training builds on the techniques learned during cavern training and includes the training needed to penetrate beyond the cavern zone and working with permanent guidelines that exist in many caves. Once intro to cave certified, a diver may penetrate much further into a cave, usually limited by 1/3 of a single cylinder, or in the case of a basic cave certification, 1/6 of double cylinders. An intro cave diver is usually not certified to do complex navigation. 
- Apprentice cave training serves as the transition from intro to full certification and includes the training needed to penetrate deep into caves working from permanent guide lines as well as limited exposure to side lines that exist in many caves. Training covers complex dive planning and decompression procedures used for longer dives. Once apprentice certified, a diver may penetrate much further into a cave, usually limited by 1/3 of double cylinders. An apprentice diver is also allowed to do a single jump or gap (a break in the guideline from two sections of mainline or between mainline and sideline) during the dive. An apprentice diver typically has one year to finish full cave or must repeat the apprentice stage.
- Full cave training serves as the final level of basic training and includes the training needed to penetrate deep into the cave working from both permanent guidelines and sidelines, and may plan and complete complex dives deep into a system using decompression to stay longer. Once cave certified, a diver may penetrate much further into a cave, usually limited by 1/3 of double cylinders. A cave diver is also certified as competent to do multiple jumps or gaps (a break in the guideline from two sections of mainline or between mainline and sideline) during the dive.
Cave diving is one of the most challenging and potentially dangerous kinds of diving or caving and presents many hazards. Cave diving is a form of penetration diving, meaning that in an emergency a diver cannot swim vertically to the surface due to the cave's ceilings, and so must swim the entire way back out. The underwater navigation through the cave system may be difficult and exit routes may be at considerable distance, requiring the diver to have sufficient breathing gas to make the journey. The dive may also be deep, resulting in potential deep diving risks.
Visibility can vary from nearly unlimited to low, or non-existent, and can go from one extreme to the other in a single dive. While a less-intensive kind of diving called cavern diving does not take divers beyond the reach of natural light (and typically no deeper than 100 feet (30 m)), and penetration not further than 200 feet (60 m), true cave diving can involve penetrations of many thousands of feet, well beyond the reach of sunlight. The level of darkness experienced creates an environment impossible to see in without an artificial form of light. Caves often contain sand, mud, clay, silt, or other sediment that can further reduce underwater visibility in seconds when stirred up.
Caves can carry strong water currents. Most caves emerge on the surface as either springs or siphons. Springs have out flowing currents, where water is coming up out of the Earth and flowing out across the land's surface. Siphons have in-flowing currents where, for example, an above-ground river is going underground. Some caves are complex and have some tunnels with out-flowing currents, and other tunnels with in-flowing currents. If currents are not properly managed, they can cause serious problems for the diver.[clarification needed]
Cave diving has been perceived as one of the more deadly sports in the world. This perception may be exaggerated because the majority of divers who have lost their lives in caves have either not undergone specialized training or have had inadequate equipment for the environment. Some cave divers have suggested that cave diving is in fact statistically much safer than recreational diving due to the much larger barriers imposed by experience, training, and equipment cost, but there is no definitive statistical evidence for this claim.
There is no reliable worldwide database listing all cave diving fatalities. Such fractional statistics as are available, however, suggest that very few divers have ever died while following accepted protocols and while using equipment configurations recognized as acceptable by the cave diving community. In the very rare cases of exceptions to this rule there have typically been unusual circumstances. Much the same can be said for other modes and applications of diving. There is little doubt that competence, good preparation and appropriate equipment reduce the risk of all hazardous activities.
Most cave divers recognize five general rules or contributing factors for safe cave diving, which were popularized, adapted and became generally accepted from Sheck Exley's 1979 publication Basic Cave Diving: A Blueprint for Survival. In this book, Exley included accounts of actual cave diving accidents, and followed each one with a breakdown of what factors contributed to the accident. Despite the unique circumstances of each individual accident, Exley found that at least one of a small number of major factors contributed to each one. This technique for breaking down accident reports and finding common causes among them is now called Accident Analysis, and is taught in introductory cave diving courses. Exley outlined a number of these resulting cave diving rules, but today these five are the most recognized:
- Training: A safe cave diver does not intentionally exceed the scope of their training. Cave diving is normally taught in stages, each successive stage focusing on more complex aspects of cave diving. Each stage of training is intended to be reinforced with actual cave diving experience to develop competence before starting training at a more complex level. Accident analysis of cave diving fatalities has shown that academic training without sufficient real world experience is not always enough in the event of an underwater emergency. By systematically building experience the diver can develop the confidence, motor skills and reflexes to remain calm and apply the appropriate procedures in an emergency. An inexperienced diver is more likely to panic than an experienced diver when confronted with a similar situation.
- Guide line: A continuous guide line is maintained at all times between the leader of a dive team and a fixed point selected outside the cave entrance in open water. Often this line is tied off a second time as a backup directly inside the cavern zone. As the dive leader lays the guideline they take great care to ensure there is appropriate tension on the line. and that it does not go into line traps. Other team members remain between the lead diver and the exit, in easy reach of the line at all times. If a silt out occurs, divers can find the line and follow it back to the cave entrance. Failure to use a continuous guide line to open water is cited as the most frequent cause of fatality among untrained, non-certified divers who venture into caves.
- Depth rules: Gas consumption, nitrogen narcosis and decompression obligation increase with depth, and the effects of nitrogen narcosis may be more critical in a cave due to the high task loading and presence of combinations of hazards. Cave divers are advised not to dive to depths exceeding the planned depth and the applicable range of their equipment and the breathing gases in use, and to keep in mind this effective difference between open water depth and cave depth. Excessive depth is frequently cited as a contributory factor in fatal incidents involving fully trained cave divers.
- Breathing gas management: The breathing gas supply must last the diver until out of the overhead environment. There are several strategies for gas management. The most common protocol is the 'rule of thirds,' in which one third of the initial gas supply is used for ingress, one third for egress, and one third to support another team member in the case of an emergency. This is a very simple method, but is not always sufficient. UK practice is to adhere to the rule of thirds, but with an added emphasis on keeping depletion of the separate air systems "balanced", so that the complete loss of any single gas supply will still leave the diver with sufficient gas to return safely. The rule of thirds makes no allowance for increased air consumption that the stress caused by the loss of an air system may induce. Dissimilar tank sizes among the divers are also not allowed for by the rule of thirds, and a sufficient reserve should be calculated for each dive. UK practice is to assume that each diver is completely independent, as in a typical UK sump there is usually nothing that a buddy can do to assist a diver in trouble. Most UK cave divers dive solo. US sump divers follow a similar protocol. The rule of thirds was devised as an approach to diving Florida's caves – they typically have high outflow currents, which help to reduce air consumption when exiting. In a cave system with little or no outflow it is prudent to reserve more air than is provided by the rule of thirds.
- Lights: Each cave diver should have at least three independent sources of light. One is considered the primary and is intended for general use during the dive. The others are considered backup lights and may be lower powered as they are not intended for exploration. Each light must have an expected burn time of at least the planned duration of the dive. If any diver loses light function so that they have fewer than three working lights, protocol requires that the dive is turned for all members of the dive team and exit started immediately.
In recent years new contributing factors were considered after reviewing accidents involving solo diving, diving with incapable dive partners, video or photography in caves, complex cave dives and cave diving in large groups. With the establishment of technical diving, the use of mixed gases—such as trimix for bottom gas, and nitrox and oxygen for decompression—reduces the margin for error. Accident analysis suggests that breathing the wrong gas for the depth or not analyzing the breathing gas properly has also led to cave diving accidents.
Cave diving requires a variety of specialized procedures, and divers who do not correctly apply these procedures may significantly increase the risk to the members of their team. The cave diving community works hard to educate the public on the risks they assume when they enter water-filled caves. Warning signs with the likenesses of the Grim Reaper have been placed just inside the openings of many popular caves in the US, and others have been placed in nearby parking lots and local dive shops.
Many cave diving sites around the world include open-water basins, which are popular open-water diving sites. The management of these sites try to minimize the risk of untrained divers being tempted to venture inside the cave systems. With the support of the cave diving community, many of these sites enforce a "no-lights rule" for divers who lack cave training—they may not carry any lights into the water with them. It is easy to venture into an underwater cave with a light and not realize how far away from the entrance (and daylight) one has swum; this rule is based on the theory that, without a light, divers will not venture beyond daylight.
In the early phases of cave diving the analysis shows that 90% of accidents were not trained cave divers; from the 2000s on the trend has reversed to 80% of accidents involving trained cave divers. Modern cave divers' capability and available technology allows divers to venture well beyond traditional training limits[clarification needed] and into actual exploration. The result is an increase of cave diving accidents, in 2011 the yearly average of 2.5 fatalities a year tripled. In 2012 fatalities reached the highest annual rate to that date at over 20.
As response to the increase in fatalities during the years 2010 onwards, the International Diving Research and Exploration Organization (IDREO) was created in order to "bring awareness of the current safety situation of Cave Diving" by listing current worldwide accidents by year and promoting a community discussion and analysis of accidents through a "Cave Diver Safety Meeting" held annually.
Equipment used by cave divers ranges from fairly standard recreational scuba configurations, to more complex arrangements which allow more freedom of movement in confined spaces, extended range in terms of depth and time, allowing greater distances to be covered in acceptable safety, and equipment which helps with navigation, in what are usually dark, and often silty and convoluted spaces.
Scuba configurations which are more often found in cave diving than in open water diving include independent or manifolded twin cylinder rigs, side-mount harnesses, sling cylinders, rebreathers and backplate and wing harnesses. Bill Stone designed and used epoxy composite tanks for exploration of the San Agustín and Sistema Huautla caves in Mexico to decrease the weight for dry sections and vertical passages.
Stage cylinders are cylinders which are used to provide gas for a portion of the penetration. They may be deposited on the bottom at the guideline on preparation dives, to be picked up for use during the main dive, or may be carried by the divers and dropped off at the line during the penetration to be retrieved on the way out.
One of the high risk hazards of cave diving is getting lost in the cave. The use of guidelines is the standard mitigation for this risk. Guidelines may be permanent or laid and recovered during the dive, using cave reels to deploy and recover the line. Permanent branch lines may be laid with a gap between the start of the branch line and the nearest point on the main line. Gap spools with a relatively short line are commonly used to make the jump.
Silt screws are short lengths of rigid tube (usually plastic) with one sharpened end and a notch or slot at the other end to secure the line, which are pushed into the silt or detritus of the cave floor as a place to tie off a guideline when no suitable natural tie-off points are available.
Diver propulsion vehicles, or Scooters, are sometimes used to extend the range by reducing the work load on the diver and allowing faster travel in open sections of cave. Reliability of the diver propulsion vehicle is very important, as a failure could compromise the ability of the diver to exit the cave before running out of gas. Where this is a significant risk, divers may tow a spare scooter.
Dive lights are critical safety equipment, as it is dark inside caves. Each diver generally carries a primary light, and at least one backup light. A minimum of three lights is recommended. The primary light should last the planned duration of the dive, as should the backup lights.
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The cave diving community is a global one, partly due to the highly specialised nature with the resulting small numbers of practitioners at a local level. Cave diving practice can differ markedly by locality.
One such difference is the use of a floating polypropylene guide line. Most cave divers in the U.S. balk at the use of any sort of floating guide line, 6 mm nylon line is the norm in UK and is regularly anchored to stones, lead weights, or whatever is needed these belays are laid clear of mud and silt. In continental Europe, thinner yet slightly buoyant line is typical. Cave diving practices in some localities may be different than those in other parts of the world because those caves require specialized techniques.
Consistency in signs and warnings also differs around the world. For example, warnings signs are rare in the UK.
Jacques-Yves Cousteau, co-inventor of the first commercially successful open circuit scuba equipment, was the world's first open circuit scuba cave diver. However, many cave divers penetrated caves prior to the advent of scuba with surface supplied breathing apparatus through the use of umbilical hoses and compressors. Scuba diving in all its forms, including cave diving, has advanced in earnest since Cousteau introduced the Aqua-Lung in 1943.
Two regions have had particular influence on cave diving techniques and equipment due to their very different cave diving environments. These are the United Kingdom, and the United States, mainly Florida.
The Cave Diving Group (CDG) was established informally in the United Kingdom in 1935 to organise training and equipment for the exploration of flooded caves in the Mendip Hills of Somerset. The first dive was made by Jack Sheppard on 4 October 1936, using a home-made drysuit surface fed from a modified bicycle pump, which allowed Shepard to pass Sump 1 of Swildon's Hole. Swildon's is an upstream feeder to the Wookey Hole resurgence system. The difficulty of access to the sump in Swildon's prompted operations to move to the resurgence, and the larger cave there allowed use of conventional "hard hat" equipment which was secured from the Siebe Gorman company. The left photograph on the standard diving dress page will give some indication of the scale of operations this entailed. In UK cave diving, the term "Sherpa" is used without a drop of irony for the people who carry the diver's gear although this has gone out of fashion; support is now more normally used, and before the development of SCUBA equipment such undertakings could be monumental operations.
Diving in the spacious third chamber of Wookey Hole led to a rapid series of advances, each of which was dignified by being given a successive number, until an air surface was reached at what is now known as "Chamber 9." Some of these dives were broadcast live on BBC radio, which must have been a quite surreal experience for both diver and audience.
The number of sites where standard diving dress could be used is clearly limited and there was little further progress before the outbreak of World War II reduced the caving community considerably. However, the rapid development of underwater warfare through the war made a lot of surplus equipment available. The CDG re-formed in 1946 and progress was rapid. Typical equipment at this time was a frogman rubber diving suit for insulation (water temperature in the UK is typically 4 °C), an oxygen diving cylinder, soda lime absorbent canister and counter-lung comprising a rebreather air system and an "AFLOLAUN", meaning "Apparatus For Laying Out Line And Underwater Navigation." The AFLOLAUN consisted of lights, line-reel, compass, notebook (for the survey), batteries, and more.
Progress was typically by "bottom walking", as this was considered less dangerous than swimming (note the absence of buoyancy controls). The use of oxygen put a depth limit on the dive, which was considerably mitigated by the extended dive duration. This was the normal diving equipment and methods until approximately 1960 when new techniques using wetsuits (which provide both insulation and buoyancy ), twin open-circuit SCUBA air systems the development of side mounting cylinders, helmet-mounted lights and free-swimming with fins. The increasing capacity and pressure rating of air bottles also extended dive durations.
In the 1970s, cave diving greatly increased in popularity among divers in the United States. However, there were very few experienced cave divers and almost no formal classes to handle the surge in interest. The result was a large number of divers trying to cave dive without any formal training. This resulted in more than 100 fatalities over the course of the decade. The state of Florida came close to banning SCUBA diving around the cave entrances. The cave diving organizations responded to the problem by creating training programs and certifying instructors, in addition to other measures to try to prevent these fatalities. This included posting signs, adding no-lights rules, and other enforcements.
In the United States, Sheck Exley was a pioneering cave diver who first explored many underwater cave systems in Florida, and many throughout the US and the world. On 6 February 1974, Exley became the first chairman of the Cave Diving Section of the National Speleological Society.
Since the 1980s, cave diving education has greatly reduced diver fatalities, and it is now rare for an agency trained diver to perish in an underwater cave. Also in the 1980s, refinements were made to the equipment used for cave diving, most importantly better lights with smaller batteries. In the 1990s, cave diving equipment configurations became more standardized, due mostly to the adaptation and popularization of the "Hogarthian Rig", developed by several North Florida cave divers, named in honor of William "Hogarth" Main, which promotes equipment choices that "keep it simple and streamlined".
Cave diving venues
- Boesmansgat is a sinkhole in the Northern Cape province has been dived to 282.6 metres depth.
- Wondergat (more of a cavern, but with a small cave section)
- Komati Springs – a flooded mine
Pawod Underwater Cave in Mactan Island, off the island of Cebu in the Central Philippines was discovered in 2001 by Dr. Alfonso Amores. The discovery opened cave diving for the locals who erstwhile could not afford the expensive course in Florida. In 2012, Jake Miranda, Dr. Alfonso Amores and Bernil H. Gastardo founded the Filipino Cave Divers (FCD). Exploration opened the freshwater caves of Casili, Balamban in Cebu Island and Enchanted River Underwater Cave in Hinatuan, Surigao del Sur in the big southern island of Mindanao. Several more caves in Mindanao have since been discovered and explored – Bababu Lake (Dinagat Island), Pamutuanan Cave (Lianga, Surigao del Sur) and Campomento Cave (Lanuza, Surigao del Sur).
West Timor, Indonesia
There are a number of freshwater filled cave sites located near and within Kupang, the principal city in West Timor and the provincial capital of East Nusa Tenggara. At least 2 sites have been described: Gua Oehani (translates as the Oehani Cave) and Gua Kristal (translates as the Crystal Cave).
As of 1999, there was no requirement to hold cave diving certification to dive either of these sites. However, competence equivalent to CDAA Advanced Cave and Cave/Advanced Cave respectively appears to be appropriate for these sites.
Australia has many spectacular water filled caves and sinkholes, but unlike the UK, most Australian cave divers come from a general ocean-diving background.
The "air-clear" water conditions experienced in the sinkholes and caves found in the Lower South East (now called the Limestone Coast) of South Australia (SA) has attracted many visiting divers with the first cave and sinkhole dives taking place in the very late 1950s. Until the mid-1980s divers generally used single diving cylinders and homemade torches, and reels, resulting in most of their explorations being limited. Mixed-gas and rebreather technologies can now be used in many sites. The area is usually known within the cave diving community as the Mount Gambier region.
A series of incidents between 1969 and 1974 in the former Lower South East of SA in which 11 divers died (including a triple and a quadruple fatality) in the following four karst features – Kilsbys Hole, Piccaninnie Ponds, Death Cave (also known as Alleyns Cave) and The Shaft – created much public comment and led to the formation of the Cave Divers Association of Australia Inc. (CDAA) in September 1973. The introduction of a testing program by the CDAA in 1974, which involved the assessment of prospective cave divers' cave diving ability led to a reduced fatality rate. In 1989, this testing system was replaced by a training system which consists of three levels of qualification – Deep Cavern, Cave and Advanced Cave. Five further deaths have occurred since 1974; two died at Piccaninnie Ponds in 1984, one person died at Kilsbys Hole in 2010, and two people died in separate incidents at Tank Cave in 2011 including noted cave diver Agnes Milowka.
During the 1980s, the Nullarbor Plain was recognized as a major cave-diving area, with one cave, Cocklebiddy, being explored for more than 6 kilometres, involving the use of large sleds to which were attached numerous diving cylinders and other paraphernalia, and which were then laboriously pushed through the cave by the divers. In more recent years divers have been utilizing compact diver-towing powered scooters, but the dive is still technically extremely challenging.
A number of other very significant caves have also been discovered during the past 20 years or so; the 10+ (Lineal) kilometre long Tank Cave near Millicent in the Limestone Coast, other very large features on the Nullarbor and the adjacent Roe Plain as well as a number of specific sites elsewhere, and nowadays the cave diving community utilizes many techniques, equipment and standards from the U.S. and elsewhere.
The CDAA is the major cave diving organisation in Australia and is responsible for the administration of cave diving at many sites. All cave diving in the Limestone Coast as well as at some New South Wales sites and the Nullarbor requires divers to be members of the CDAA, whether in the capacity of a visitor or a trained and assessed member. A number of other organisations participate in cave diving activities within Australia. The Australian Speleological Federation Cave Diving Group which was formed in 2005 coordinates projects focused on exploration and mapping at sites throughout Australia. The following diving training organisations offer courses in various aspects of cave diving via instructors either resident in Australia or visiting from overseas – Global Underwater Explorers, International Association of Nitrox and Technical Divers and Technical Diving International.
Bosnia and Herzegovina
Una National Park, in the Bihac municipality, encompasses the valley of the Una River and is a hotspot for speleological research expeditions. Thanks to the area’s position between the end of two large karst systems: Plješivica and Grmeč, the river abounds with shallow water sources and a significant number of underwater caves. So far, 10 large caves have been registered, only five of which have been partially examined. Divers who have explored Klokot Spring have managed to reach a depth of 104.5 metres (343 ft), though the bottom of the spring has yet to be touched. Another cave has been explored to the length of 150 metres (490 ft), and is said to be an ideal destination for cave divers in training.
Hranice Abyss, or Hranická Propast, is the world's deepest known underwater cavity. It beat the previous record holder, Pozzo del Merro in Italy, by 12 metres (39 ft). Polish explorer Krzysztof Starnawski, who has explored the upper parts of the flooded fissure for decades, has determined the depth to be at least 404 metres (1,325 ft) . As of 2017, divers have yet to reach the bottom of the cave.
Molnár János Cave is an active thermal water cave located on the western banks of the Danube river. Belonging to the Buda Hills karst system, which is home to many of Budapest's famed hot springs, the temperature of the water sits between 20–28 °C (68–82 °F) year-round.  Whilst the first underwater explorations began in the 1950s, the cave wasn't successfully chartered until the 70s and 80s when a group of divers mapped out more than 400 metres (1,300 ft) of the cave system. The cave, which contains a labyrinth passages, currently stretches to over 6 km (3.7 mi) though there are many more untouched sections to explore.
After the death of a diver in 2011, the Hungarian government imposed strict rules for diving. To dive at Molnár János, divers must have at least an intro-to-cave-diving certification or they cannot dive in the tunnels; they must have dive insurance that covers cave diving; and they must have air supplied in the form of double tanks, stages or a rebreather. In addition, each dive is accompanied by a guide, to minimize the risk of a wrong turn. 
Silfra is a fissure between the North American and Eurasian tectonic plates in Thingvellir National Park. It is the only place in the world where you can dive or snorkel directly in a crack between two tectonic plates. The tunnel from the entrance of Silfra to the cave is commonly known as ‘The Tunnel’, due to the strong current that ‘flushes’ divers attempting to swim through. This tunnel is rarely dived due to the dangerous head-first vertical dive needed to pass through. The deepest part of the cave is at least 63 metres (207 ft) deep, and has only been dived by a few people because of the narrow and unstable passages.
In the north west of Sardinia, close to Porto Conte bay, Alghero territory, there are more than 300 caves above and below water, with about 30 large, and many smaller, underwater sea caves in the limestone cliffs of Capo Caccia and Punta Giglio. The Nereo Cave is the most important and is considered the largest in the Mediterranean Sea. On the east side of Sardinia there are many underwater cave systems starting from the Gennargentu Mountains, with underwater rivers. There is a fresh water cave of more than 110 m (360 ft) depth in this area.
Cave divers occasionally visit Jordbrugrotta which is the most dived cave in Scandinavia. Most of the other approximately 200 caves in Rana are not suitable for diving, and formation of caves has been limited due most of the rock being granite. Another diveable cave nearby is Litjåga. Damming of Kallvatnet made diving in Jordbrugrotta possible. There have been multiple cases of fatalities and injuries among visiting cave divers, but the accident rate is not out of proportion with the number of divers. The cave's passages were formed by the river Plura's flow through porous limestone. Rock formations include marble. Visibility in the cave is good.
- Slovak Opal Mine, near Prešov. The mine consists of around 22 km (14 mi) of tunnels; with some measuring as deep as 150 metres (490 ft).
UK requirements are generally that all people wishing to take up cave diving must be competent cavers before they start cave diving. This is primarily because most British cave dives are at the far end of dry caves. There are individuals that begin cave diving directly from the recreational diving, but they represent a minority in the UK, and represent only a few percent of the Cave Diving Group (CDG).
The caves and caverns of Grand Bahama contain an immense underwater cavern with a vast flooded labyrinth of caverns, caves and submerged tunnels that honeycomb the entire island of Grand Bahama and the surrounding sea bed. The inland caves are not abundant with life, but do contain creatures living in the caves other than the migrating gray snappers. Residents of these caves include a type of blind cave fish and remipedia that don't pose any threat to cave divers.
The caves in the Bahamas were formed during the last ice age. With much of the Earth's water held in the form of glacial ice, the sea level fell hundreds of feet, leaving most of the Bahama banks, which are now covered in water, high and dry. Rain falling on the most porous limestone slowly filtered down to sea level forming a lens where it contacted the denser salt water of the ocean permeating the spongy lime stone. The water at the interface was acidic enough to dissolve away the limestone and form the caves. Then, as more ice formed and the sea level dropped even further, the caves became dry and rainwater dripping through the ceiling over thousands of years created the incredible crystal forests of stalagmites which now decorate the caves. Finally, when the ice melted and the sea level rose, the caves were reclaimed by the sea.
There is a growing number of known water filled caves in the Dominican Republic, spread all over the island. Regions with underwater caves include Santo Domingo, Pedernales, Cabrera, Bayahibe and others.
Active exploration is being conducted by the Dominican Republic Speleological Society which is working together with local institutions as well as international scientists to further explore all the cave systems possibilities and focusing in the preservation.
The longest known cave in the island is El Toro which is about 6,000 ft (1,800 m) in length.
The best known caves in the island are Cueva Taina, El Tildo, El Chicho and El Dudu, which have easy access to the water and with a good level of safety outside of the water as they are in private properties or national parks.
Cave diving is playing a very important role in science in the DR, in the last 3 years the DRSS team together with international scientists and "Museo del Hombre Dominicano" has found a new species of cave bacteria, a number of new and extinct bat species, the first evidence of extinct crocodiles in the DR, fossil snakes, birds, sloths and remains of long extinct monkeys and other ancient cave life. DRSS has cataloged over 120 new springs all over the island in which many have caverns and cave systems attached.
Safety concerns in Dominican Republic
An important safety issue on the island is untrained cave diving, principally led by unprofessional dive stores and guides risking their customers' lives. Unprofessional and unsafe practices of dive guides may be reported to the relevant dive agency and to the Dominican Republic Speleological Society.
While there is great potential for cave diving in the continental karst throughout Mexico, the majority of cave diving in Mexico occurs in the Yucatán Peninsula. While there are thousands of deep pit cenotes throughout the Yucatán Peninsula including in the states of Yucatán and Campeche, the extensive sub-horizontal flooded cave networks for which the peninsula is known are essentially limited to a 10 km wide strip of the Caribbean coastline in the state of Quintana Roo extending south from Cancún to the Tulum Municipality and the Sian Ka'an Biosphere Reserve, although some short segments of underwater cave have been explored on the north-west coast (Yucatán State).
In the Yucatán Peninsula, any surface opening where groundwater can be reached is called cenote, which is a Spanish form of the Maya word d'zonot. The cave systems formed as normal caves underwater, but upper sections drained becoming air filled during past low sea levels. During this vadose, or air filled state, abundant speleothem deposits formed. The caves and the vadose speleothem were reflooded and became hydraulically reactivated as rising sea levels also raised the water table. These caves are therefore polygenetic, having experienced more than one cycle of formation below the water table. Polygenetic coastal cave systems with underwater speleothem are globally common, with notable examples being on the Balearic Islands (Mallorca, Menorca) of Spain, the islands of the Bahamas, Bermuda, Cuba, and many more.
The underwater speleothems in the Yucatán Peninsula are fragile. If a diver accidentally breaks a formation, it will not reform as long as the cave is underwater so active cave conservation diving techniques are necessary.[clarification needed] The Quintana Roo caves are extremely complex with anastomotic interconnected passages. When diving through the caves, they appear to have many offshoots and junctions, requiring careful navigation with permanent tees or the implementation of jumps in the guideline.
The beginning of the 1980s brought the first cave divers from the U.S. to the Yucatán Peninsula, Quintana Roo to explore cenotes such as Carwash, Naharon and Maya Blue, and to central Mexico where resurgence rivers such as Rio Mante, and sinkholes such as Zacaton were documented.
In the Yucatán, the 1980s ended with the discoveries of the Dos Ojos and Nohoch Nah Chich cave systems which led into a long competition of which exploration team had the longest recorded underwater cave system in the world at the time.
By the mid-1990s a push into the central Yucatán Peninsula discovered a large number of deep sinkholes, or pit cenotes, such as Sabak Ha and Utzil, and deep caves such as Chacdzinikche, Dzibilchaltun and Kankirixche that have since been explored and mapped. These deep caves of the central Yucatán remain largely unexplored due to the number of cenotes in the State of Yucatán, and the depths, which require technical diving techniques or rebreathers. At the end of the last millennium closed circuit rebreathers were used to explore these caves.
By the end of the 1990s, The Pit in the Dos Ojos cave system located 5.8 km from the Caribbean coast had been discovered, and by 2008 it had been dived to 119 m deep. At that time, technical diving and rebreather equipment and techniques became commonplace.
By the turn of the millennium the longest underwater cave system at that time, Ox Bel Ha was established by cave diving explorers whose combined efforts and information helped join segments of previously explored caves. The use of hand held GPS technology and aerial and satellite images for reconnaissance during exploration became common. New technology such as improved rebreathers and diver propulsion vehicles (DPVs) became available and were utilized for longer penetration dives. In January 2013, Ox Bel Ha included 242 km of underwater passage (see QRSS for current statistics).
Active exploration continues in the new millennium. Most cave diving exploration is now conducted on the basis of "mini projects" lasting 1 – 7 days, and occurring many times a year, and these may include daily commutes from home to jungle dive base camps located within 1 hour from road access.
Starting in 2006 a number of large previously explored and mapped cave systems have been shown to be connected using sidemount and often no-mount cave diving techniques to pass through tight cave passages, indicating the largest known connected underwater cave system on the planet, Sac Actun, which is 220 km long (see QRSS for current statistics).
Many cave maps have been published by the Quintana Roo Speleological Survey (QRSS).
Central and Northern Florida
The largest and most active cave diving community in the United States is in north-central Florida. The North Floridan Aquifer expels groundwater through numerous first-magnitude springs, each providing an entrance to the aquifer's labyrinthine cave system. These high-flow springs have resulted in Florida cave divers developing special techniques for exploring them, since some have such strong currents that it is impossible to swim against them.
The longest known underwater cave system in the United States, The Leon Sinks cave system, near Tallahassee, Florida, has multiple interconnected sinks and springs spanning two counties (Leon & Wakulla). One main resurgence of the system, Wakulla Springs, is explored exclusively by a very successful and pioneering project called the Woodville Karst Plain Project (WKPP), although other individuals and groups like the US Deep Cave Diving Team, have explored portions of Wakulla Springs in the past.
One deep underwater cave in the United States is Weeki Wachee Spring. Due to its strong outflow, divers have had limited success penetrating this first magnitude spring until 2007, when drought conditions eased the out-flowing water allowing team divers from Karst Underwater Research to penetrate to depths of 400 feet (120 m) The current deepest known underwater cave in the United States as of 2013 is Phantom Springs Cave located in west Texas. Phantom Springs has been explored down to 462 feet (141 m) in water filled cave passages.
The Florida caves are formed from geologically young limestone with moderate porosity. The absence of speleothem decorations which can only form in air filled caves, indicates that the flooded Florida caves have a single genetic phase origin, having remained water filled even during past low sea levels. In plan form, the caves are relatively linear with a limited number of side passages allowing for most of the guidelines to be simple paths with few permanent tees. It is common practice for cave divers in Florida to joint a main line with a secondary line using a jump reel when exploring side passages, in order to maintain a continuous guideline to the surface.
In Brazil there is cavern diving in Chapada da Diamantina, in Bahia state; Bonito, in Mato Grosso do Sul state; and Mariana, where there is also cave diving (visiting Mina da Passagem), in Minas Gerais state. For cave diving in Mariana a cave diver certification will be required.
Notable cave divers:
- David Apperley
- Craig Challen
- Sheck Exley
- Martyn Farr
- Nuno Gomes
- Paul Hosie
- William Stone
- Andrew Wight
- John Volanthen
- Rick Stanton
- Disappearance of Ben McDaniel, occurred while cave diving without certification
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- Atlas of Caves Worldwide caveatlas.com
- Florida Cave & Cavern List caveatlas.com
- Article Cave Diving Group, UK, 28 March 2010
- Cave Diving Group manual Cave Diving Group, UK,
- Woodville Karst Plain Project non profit organization about North Florida's underwater cave systems
- Wakulla system Todd Kincaid, University of Wyoming
- Global Underwater Explorers non profit organization in Florida, education and exploration; Project Baseline, online spatial database of global underwater conditions
- International Underwater Cave Rescue and Recovery (IUCRR) International non profit organization registered in Florida
- brief cave diving article Divetime.com, 2009-03-20
- A cavern dive video YouTube
- Cave Diving Down Under (Australia) social media site for cave diving in Australia
- Dominican Republic Speleological Society
- Deep in the dark Social media site for cave divers, locations and conditions of caves, articles on learning to cave dive