Deep sea

The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combination of low temperatures, darkness and high pressure. The deep sea is considered the least explored Earth biome as the extreme conditions make the environment difficult to access and explore. Organisms living within the deep sea have a variety of adaptations to survive in these conditions. Organisms can survive in the deep sea through a number of feeding methods including scavenging, predation and filtration, with a number of organisms surviving by feeding on marine snow. Marine snow is organic material that has fallen from upper waters into the deep sea. In 1960, the bathyscaphe Trieste descended to the bottom of the Mariana Trench near Guam, at , the deepest known spot in any ocean. If Mount Everest () were submerged there, its peak would be more than beneath the surface. After the Trieste was retired, the Japanese remote-operated vehicle (ROV) Kaikō was the only vessel capable of reaching this depth until it was lost at sea in 2003. In May and June 2009, the hybrid-ROV Nereus returned to the Challenger Deep for a series of three dives to depths exceeding . Natural light does not penetrate the deep ocean, with the exception of the upper parts of the mesopelagic. Since photosynthesis is not possible, plants and phytoplankton cannot live in this zone, and as these are the primary producers of almost all of earth's ecosystems, life in this area of the ocean must depend on energy sources from elsewhere. Except for the areas close to the hydrothermal vents, this energy comes from organic material drifting down from the photic zone. The sinking organic material is composed of algal particulates, detritus, and other forms of biological waste, which is collectively referred to as marine snow. Because pressure in the ocean increases by about 1 atmosphere for every 10 meters of depth, the amount of pressure experienced by many marine organisms is extreme.