Hadal zone

The hadal zone ranges from around 6 to 11 km (3.7 to 6.8 mi; 20,000 to 36,000 ft) below sea level, and exists in long, narrow, topographic V-shaped depressions.

[1][2] The cumulative area occupied by the 46 individual hadal habitats worldwide is less than 0.25% of the world's seafloor, yet trenches account for over 40% of the ocean's depth range.

During the early 1950s, the Danish Galathea II and Soviet Vityaz expeditions separately discovered a distinct shift in the life at depths of 6,000–7,000 m (20,000–23,000 ft) not recognized by the broad definition of the abyssal zone.

There are high levels of endemism, and noteworthy examples of gigantism in amphipods, mysids, and isopods and dwarfism in nematodes, copepods, and kinorhynchs.

Trench communities do, nevertheless, display a contrasting degree of intra-trench endemism and inter-trench similarities at a higher taxonomic level.

[5] Only a relatively small number of fish species are known from the hadal zone, including certain grenadiers, cutthroat eels, pearlfish, cusk-eels, snailfish and eelpouts.

[18][19] Some invertebrates do occur deeper, such as bigfin squid,[20][21] certain polynoid worms, myriotrochid sea cucumbers, turrid snails and pardaliscid amphipods in excess of 10,000 m (33,000 ft).

[24] Otherwise the first link in the hadal food web are heterotroph organisms that feed on marine snow, both fine particles and the occasional carcass.

[28] The first manned exploration to reach Challenger Deep, the deepest known part of the ocean located in the Mariana Trench, was accomplished in 1960 by Jacques Piccard and Don Walsh.

[31] The descent of the Deepsea Challenger reached a depth of 10,908 metres (35,787 ft), slightly less than the deepest dive record set by Piccard and Walsh.