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, with the extreme conditions making 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 10,911 m (35,797 ft; 6.780 mi), the deepest known spot in any ocean. If Mount Everest (8,848 m or 29,029 ft or 5.498 mi) were submerged there, its peak would be more than 2 km (1.2 mi) 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 10,900 m (35,800 ft; 6.8 mi).