The deep water in ocean trenches. The name is derived from the Greek Άιδης (Haidēs), Hades, the classical Greek underworld. This zone is mostly unknown and very few species are known to live here (in the open areas). However, many organisms live in hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000m (19,685 ft), whether in a trench or not.