The hadal zone (named after the realm of Hades, the underworld in Greek mythology), also known as the hadopelagic zone and trench zone, is the delineation for the deepest trenches in the ocean. This zone is found from a depth of around 6,000 metres (20,000 ft) to the bottom of the ocean. The hadal zone has low population and low diversity of marine life.
It is believed that most life at this depth is sustained by marine snow or the chemical reactions around thermal vents. The intense pressure and the lack of light create hostile living conditions and few species are able to exist here. As no sunlight reaches this layer of the ocean, deep sea creatures have reduced eyesight, with very large eyes for receiving only bioluminescent flashes. Most of the bottom-dwelling creatures lack any pigmentation, since coloration is not useful in an environment with no light.
Organisms from this zone will die in the zones where pressure is lower.  The most common organisms include jellyfish, viperfish, tube worms and sea cucumbers. The hadal zone can reach far below 6,000 meters (20,000 feet) deep; the deepest known extends to 10,911 meters (35,814 ft). At such depths (for example, at 36,000 feet below sea level) the pressure in the Hadal zone exceeds 1,100 standard atmospheres (110 MPa; 16,000 psi).
In 1960, Jacques Piccard and Don Walsh reached the bottom of the Mariana Trench, the deepest known trench on Earth, and observed life. James Cameron also reached the bottom in 2012 using the Deepsea Challenger.
Notes and references
- Death of a Hadal Deep-Sea Bacterium After Decompression. February 1, 2007.
- Meeresboden - down under. February 1, 2007. (German)
- "NOAA Ocean Explorer: History: Quotations: Soundings, Sea-Bottom, and Geophysics". NOAA, Office of Ocean Exploration and Research. Retrieved 2010-03-23.
- ThinkQuest. February 1, 2007.
- Than, Ker (March 25, 2012). "James Cameron Completes Record-Breaking Mariana Trench Dive". National Geographic Society.
- Forscher filmen lebende Fische in Rekordtiefe (In German) from Spiegel 10/09/2008 about an expedition filming fish at a depth of more than 7,000 m
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