The deepest named ocean zone, lying mainly within oceanic trenches, is the hadal zone.
Also called the hadopelagic zone, it generally begins below about 6,000 metres and occupies the deepest parts of trenches. The name comes from Hades, the ancient Greek underworld, reflecting the zone’s darkness and extreme environment.
The hadal zone is not simply a deeper version of the abyssal zone. The abyssal zone is usually defined as about 3,000–6,000 metres, while hadal environments occur primarily in long, narrow, V-shaped depressions created by plate subduction. Most hadal habitats are in the Pacific Ocean.
Conditions include total absence of sunlight, near-freezing water, scarce food, and pressures exceeding 1,000 atmospheres in the deepest trenches. Yet hadal ecosystems support amphipods, sea cucumbers, worms, bivalves, snailfish in shallower parts, and other highly adapted organisms.