The deep sea is the part of the ocean below a depth of 200 metres that is in permanent darkness. The abyss, with an average depth of 3.8 kilometres, is the most common environment on Earth.
The sea floor is covered by vast plains of soft sediments made up of fine detritus and particles that drift down from the surface. The water is cold because it comes from the polar regions, and its weight puts considerable pressure on life at the bottom of the sea.
Although the deep sea floor has no plants, our studies show that it is extremely diverse in animal species, often having many more than shallow water. For example, a shallow water area may have only 10 - 20 species of isopod crustaceans, while some deep sea habitats have more than 100 species.
Deep sea animals are similar worldwide. However, most species are highly endemic and limited to small parts of large, seemingly continuous ocean basins.