From the Latin ultra - over, more and the Greek abyssos - bottomless.

Ultraabyssal - the sea floor above 5000 m. The total area is less than 1.5% of the sea floor. Living conditions do not differ significantly from those in the abyssal zone, except for the hydrostatic pressure of 60-110 MPa, which creates the ecological isolation of the ultraabyssal zone from the surrounding areas of the sea floor and determines the peculiarity of its fauna (about 60% of species are endemic). Only various barophilic bacteria and 700-800 animal species have adapted to life in the ultraabyss. At depths of more than 9 thousand metres there are only a few dozen species (mainly foraminifera, actinia of the Galatheanthemidae family, polychaetes, echiurids, equal-legged crabs, bocoplaves, molluscs, holothurians, sea lilies), and at depths of more than 10 thousand metres there are only about 20 species (all endemic).

A part of the ocean more than 5,000 metres deep, mostly in deep-sea troughs. Because of its great depth, the ultraabyssal fauna is sparse (less than 800 species of animals and barophilic bacteria). With decreasing depth, the fauna becomes dramatically poorer:

At depths above 9000 metres, only a few dozen species remain (foraminifera, polychaete worms, equal-legged crabs, molluscs, holothurians);

after 10 kilometres, there are only 20 species, all endemic to the ultraabyssal zone (the zone is generally rich in endemic species, there are more than 60).