Deep sea

The deep sea or deep layer is the lowest layer in the ocean, existing below the thermocline and above the seabed, at a depth of 1000 fathoms (1800 m) or more. Little or no light penetrates this part of the ocean and most of the organisms that live there rely for subsistence on falling organic matter produced in the photic zone. For this reason scientists once assumed that life would be sparse in the deep ocean but virtually every probe has revealed that, on the contrary, life is abundant in the deep ocean.

In 1960 the Bathyscaphe Trieste descended to the bottom of the Mariana Trench near Guam, at 35,798 feet or 6.77 miles (10,911 meters), the deepest known spot in any ocean. If Mount Everest (8,848 metres) were submerged there, its peak would be more than a mile beneath the surface. The Trieste was retired and for a while the Japanese remote-operated vehicle (ROV) Kaikō was the only vessel capable of reaching this depth. It was lost at sea in 2003. In May and June 2009, the hybrid-ROV (HROV) Nereus returned to the Challenger Deep for a series of three dives to depths exceeding 10900 meters.