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Historically, the hadal zone was not recognized as distinct from the [[abyssal zone]], although the deepest sections were sometimes called "ultra-abyssal". During the early 1950s, the Danish ''[[Galathea expeditions#Second expedition|Galathea II]]'' and Soviet ''[[RV Vityaz (1939)|Vityaz]]'' expeditions separately discovered a distinct shift in the life at depths of {{Convert|6000-7000|m|ft|abbr=on}} not recognized by the broad definition of the abyssal zone.<ref>{{cite journal|author=Wolff|first=Torben|year=1959|title=The hadal community, an introduction|journal=[[Deep Sea Research]]|volume=6|pages=95–124|bibcode=1959DSR.....6...95W|doi=10.1016/0146-6313(59)90063-2}}</ref><ref name="Jamieson2010">{{Cite journal|last1=Jamieson|first1=Alan J.|author-link=Alan Jamieson|last2=Fujii|first2=Toyonobu|last3=Mayor|first3=Daniel J.|last4=Solan|first4=Martin|last5=Priede|first5=Imants G.|year=2010|title=Hadal trenches: the ecology of the deepest places on Earth (Review article)|url=http://izt.ciens.ucv.ve/ecologia/Archivos/ECO_POB%202010/ECOPO7_2010/Jamieson%20et%20al%202010.pdf|url-status=dead|journal=[[Trends in Ecology and Evolution]]|volume=25|issue=3|pages=190–197|doi=10.1016/j.tree.2009.09.009|archive-url=https://web.archive.org/web/20171225035341/http://izt.ciens.ucv.ve/ecologia/Archivos/ECO_POB%202010/ECOPO7_2010/Jamieson%20et%20al%202010.pdf|archive-date=25 December 2017|access-date=9 April 2017|pmid=19846236}}</ref> The term "hadal" was first proposed in 1956 by [[Anton Frederik Bruun]] to describe the parts of the ocean deeper than {{Convert|6000|m|ft|abbr=on}}, leaving abyssal for the parts at {{Convert|4000-6000|m|ft|abbr=on}}.<ref name="Bruun1956">{{cite journal|author=Bruun|first=Anton Frederik|author-link=Anton Frederik Bruun|date=16 June 1956|title=The Abyssal Fauna: Its Ecology, Distribution and Origin|journal=[[Nature (journal)|Nature]]|volume=177|issue=4520|pages=1105–1108|bibcode=1956Natur.177.1105B|doi=10.1038/1771105a0|s2cid=4182886}}</ref> The name refers to [[Hades]], the ancient Greek god of the [[Greek underworld|underworld]].<ref name=Bruun1956/> About 94% of the hadal zone is found in [[subduction]] trenches.<ref>[https://academic.oup.com/icesjms/article/79/4/1048/6576454?login=false Exponential growth of hadal science: perspectives and future directions identified using topic modelling]</ref>
Depths in excess of {{Convert|6000|m|ft|abbr=on}} are generally in [[ocean trench]]es, but there are also trenches at shallower depths. These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal.<ref>{{cite book|title=The First Global Integrated Marine Assessment, World Oceans Assessment I|author=United Nations|publisher=Cambridge University Press|year=2017|isbn=978-1-316-51001-8|page=904|lccn=2017287717}}</ref><ref name="Jamieson2015">{{cite book|title=The Hadal Zone: Life in the Deepest Oceans|author=Jamieson|first=Alan|author-link=Alan Jamieson|publisher=Cambridge University Press|year=2015|isbn=978-1-107-01674-3|pages=18–21, 285–318|lccn=2014006998}}</ref><ref name="Jamieson2011">{{cite book|title=eLS|author=Jamieson|first=Alan J.|author-link=Alan Jamieson|publisher=John Wiley & Sons, Ltd|year=2011|isbn=978-0470016176|chapter=Ecology of Deep Oceans: Hadal Trenches|doi=10.1002/9780470015902.a0023606}}</ref> Although the hadal zone has gained widespread recognition and many continue to use the first proposed limit of {{Convert|6000|m|ft|abbr=on}}, it has been observed that {{Convert|6000-7000|m|ft|abbr=on}} represents a gradual transition between the abyssal and hadal zones,<ref name="Jamieson2011" /> leading to the suggestion of placing the limit in the middle, at {{convert|6500|m|ft|abbr=on}}. Among others, this intermediate limit has been adopted by [[UNESCO]].<ref name="Roff2011">{{cite book|title=Marine Conservation Ecology|last1=Roff|first1=John|last2=Zacharias|first2=Mark|publisher=Earthscan|year=2011|isbn=978-1-84407-884-4}}</ref><ref name="UNESCO2009">{{cite book|url=https://unesdoc.unesco.org/ark:/48223/pf0000182451|title=Global Open Oceans and Deep Seabed (GOODS) – Biogeographic Classification|publisher=UNESCO|year=2009|editor-last=Vierros|editor-first=Marjo|series=IOC Technical Series|location=Paris|access-date=23 December 2017|editor-last2=Cresswell|editor-first2=Ian|editor-last3=Escobar Briones|editor-first3=Elva|editor-last4=Rice|editor-first4=Jake|editor-last5=Ardron|editor-first5=Jeff}}</ref> Similar to other depth ranges, the [[fauna]] of the hadal zone can be broadly placed into two groups: the hadobenthic species (compare [[benthic]]) living on or at the seabottom/sides of trenches, and the hadopelagic species (compare [[pelagic]]) living in the open water.<ref>{{cite book|title=The Living Ocean: Understanding and Protecting Marine Biodiversity|author=Thorne-Miller|first1=Boyce|last2=Catena|first2=John|publisher=John Wiley & Sons|year=1999|isbn=1-55963-678-5|edition=Second|page=57}}</ref><ref name="Meadows1988">{{cite book|title=An Introduction to Marine Science|last1=Meadows|first1=P.S.|last2=Campbell|first2=J.I.|publisher=Wiley|year=1988|isbn=978-0-470-20951-6|edition=2nd|series=Tertiary Level Biology|page=7|lccn=87020603}}</ref>
==Ecology==
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