A term in oceanography: “hadal zone” for deep-sea regions 6,000 meters below the surface

The hadal zone is the deepest zone of the ocean layering scheme. It is part of the abyssal zone that begins at  3,000 m below the ocean surface. The abyssal zone consists of the abyssopelagic zone (between 3,000 and 4,000 m), the hadalpelagic zone (between 4,000 and 6,000 m) and the hadal zone below 6,000 m [1,2]. Hadal ocean habitats are found, for example, in the Mariana Trench near the island of Guam (with Earth's deepest ocean-floor point almost 11,000 m below the surface) and the Kermadec Trench northeast of New Zealand.

The hadal zone is named after the Greek god Hades— the Lord of the Underworld and ruler of the dead [3]. Habitats of the hadal zone are dark and otherworldly, but not dead:  many fish and various marine invertebrate phyla are represented in this cold, high-pressure and oxygen-depleted or oxygen-devoid environment [4].

A Hadal Ecosystem Studies project, fittingly dubbed HADES, is planned for the next and the following years to explore the hadal life forms of the Kermadec Trench  [2]. Hadal high-tech tools (advanced imaging technology and a deep-driving Hybrid Remotely Operated Vehicle called Nereus) will play a major role in studying the biology and geology in the depth of the trench. HADES is an international collaboration with the goal to systematically study biodiversity and adaptions of life in deep ocean trenches [5].

Keywords:  marine science, expedition, ocean floor, Greek mythology.

References and more to explore
[1] The Hadal Zone Deep-sea Trenches, figure by Jeff Drazen, 2002 [cmbc.ucsd.edu/Students/Current_Students/SIO277/Trenches.pdf].
[2] Jane J. Lee: Ocean's Deep, Dark Trenches to Get Their Moment in the Spotlight. Science April 13, 2012, 336 (6078), pages 141and 143. DOI: 10.1126/science.336.6078.141.
[3] N. S. Gill: Hades - Greek God of the Underworld. About.com Guide [ancienthistory.about.com/cs/grecoromanmyth1/p/Hades.htm].
[4]  Nelson, R. 2009. "Deep Sea Biome" UntamedScience. Accessed May 13, 2012 [www.untamedscience.com/biology/world-biomes/deep-sea-biome].
[5] Aberdeen scientists in major study of deep sea life. Communications Team, Office of External Affairs, University of Aberdeen [www.abdn.ac.uk/news/archive-details-11844.php].