Thehadal zone, also known as thehadopelagic zone, is thedeepest region of theocean, lying withinoceanic trenches. The hadal zone ranges from around 6 to 11 km (3.7 to 6.8 mi; 20,000 to 36,000 ft) belowsea level, and exists in long, narrow, topographic V-shaped depressions.^[1][2]

The total area occupied by the 46 individual hadalhabitats worldwide is less than 0.25% of the world'sseafloor, yet trenches account for over 40% of the ocean's depth range.^[3] Most hadal habitats are found in thePacific Ocean, the largest and deepest of the conventional oceanic divisions.^[3]

Historically, the hadal zone was not recognized as distinct from theabyssal zone, although the deepest sections were sometimes called "ultra-abyssal". During the early 1950s, the DanishGalathea II and SovietVityaz 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.^[4][5] The term "hadal" was first proposed in 1956 byAnton Frederik Bruun to describe the parts of the ocean deeper than 6,000 m (20,000 ft), leaving abyssal for the parts at 4,000–6,000 m (13,000–20,000 ft).^[6] The name refers toHades, the ancient Greek god of theunderworld.^[6] About 94% of the hadal zone is found insubduction trenches.^[7]

Depths in excess of 6,000 m (20,000 ft) are generally inoceanic trenches, but there are also trenches at shallower depths. These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal.^[8][9][10] Although the hadal zone has gained widespread recognition and many continue to use the first proposed limit of 6,000 m (20,000 ft), it has been observed that 6,000–7,000 m (20,000–23,000 ft) represents a gradual transition between the abyssal and hadal zones,^[10] leading to the suggestion of placing the limit in the middle, at 6,500 m (21,300 ft). Among others, this intermediate limit has been adopted byUNESCO.^[11][12] Similar to other depth ranges, thefauna of the hadal zone can be broadly placed into two groups:hadobenthic species (comparebenthic zone) living on or at the seabottom/sides of trenches, andhadopelagic species (comparepelagic zone) living in open water.^[13][14]

The deepest ocean trenches are considered the least explored and most extreme marineecosystems. They are characterized by complete lack of sunlight, low temperatures, nutrient scarcity, and extremely high hydrostatic pressures. The major sources of nutrients and carbon are fallout from upper layers, drifts of fine sediment, and landslides. Most organisms are scavengers anddetrivores. As of 2020, over 400species are known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. There are high levels ofendemism, and noteworthy examples ofgigantism inamphipods,mysids, andisopods anddwarfism innematodes,copepods, andkinorhynchs.^[15]

Marine life decreases with depth, both inabundance andbiomass, but there is a wide range ofmetazoan organisms in the hadal zone, mostlybenthos, includingfish,sea cucumber,bristle worms,bivalves,isopods,sea anemones,amphipods,copepods,decapod crustaceans andgastropods. Most of these trench communities probably originated in theabyssal plains. Although they have evolved adaptations to high pressure and low temperatures such as lower metabolism, intra-cellular protein-stabilisingosmolytes, and unsaturated fatty acids in cell membranephospholipids, there is no consistent relationship between pressure and metabolic rate in these communities. Increased pressure can instead constrain theontogenic or larval stages of organisms. Pressure increases ten-fold as an organism moves from sea level to a depth of 90 m (300 ft), whilst pressure only doubles as an organism moves from 6,000 to 11,000 m (20,000 to 36,000 ft).^{[citation needed]}

Over ageological time scale, trenches can become accessible as previously stenobathic (limited to a narrow depth range) fauna evolve to become eurybathic (adapted to a wider range of depths), such asgrenadiers andnatantian prawns. Trench communities do, nevertheless, display a contrasting degree of intra-trenchendemism and inter-trench similarities at a highertaxonomic 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 andeelpouts.^[16][17] Due to the extreme pressure, the theoretical maximum depth for vertebral fish may be about 8,000–8,500 m (26,200–27,900 ft), below whichteleosts would behyperosmotic, assumingtrimethylamine N-oxide requirements follow the observed approximate linear relationship with depth.^[18][19] Some invertebrates do occur deeper, such asbigfin squid,^[20][21] certainpolynoid worms,myriotrochid sea cucumbers,turrid snails andpardaliscid amphipods in excess of 10,000 m (33,000 ft).^[9] In addition, giantprotists known asXenophyophora (foraminifera) live at these depths.^[22]

The only knownprimary producers in the hadal zone arecertain bacteria that are able to metabolizehydrogen andmethane released by rock and seawater reactions (serpentinization),^[23] orhydrogen sulfide released fromcold seeps. Some of these bacteria aresymbiotic, for example living inside the mantle of certainthyasirid andvesicomyid bivalves.^[24] Otherwise the first link in the hadalfood web areheterotroph organisms that feed onmarine snow, both fine particles and the occasional carcass.^[23][25]

The hadal zone can reach far below 6,000 m (20,000 ft) deep; thedeepest known extends to 10,911 m (35,797 ft).^[26] At such depths, the pressure in the hadal zone exceeds 1,100standard atmospheres (110 MPa; 16,000 psi). Lack of light and extreme pressure makes this part of the ocean difficult to explore.

The exploration of the hadal zone requires the use of instruments that are able to withstand pressures of up to a thousand or more atmospheres. A few haphazard and non-standard tools have been used to collect limited, but valuable, information about the basic biology of a few hadal organisms.^[27] Manned and unmannedsubmersibles, however, can be used to study the depths in greater detail. Unmanned robotic submersibles may beremotely operated (connected to the research vessel by a cable) orautonomous (freely moving). Cameras and manipulators on submersibles allow researchers to observe and take samples of sediment and organisms. Failures of submersibles under the immense pressure at hadal zone depths have occurred.HROV Nereus is thought to have imploded at a depth of 9,990 meters while exploring theKermadec Trench in 2014.^[28]

The first manned exploration to reachChallenger Deep, the deepest known part of the ocean located in theMariana Trench, was accomplished in 1960 byJacques Piccard andDon Walsh.^[29] They reached a maximum depth of 10,911 metres (35,797 ft) in the bathyscapheTrieste.^[30][27]

James Cameron also reached the bottom of Mariana Trench in March 2012 using theDeepsea Challenger.^[31] The descent of theDeepsea Challenger reached a depth of 10,908 metres (35,787 ft), slightly less than the deepest dive record set by Piccard and Walsh.^[32] Cameron holds the record for the deepest solo dive.^[30]

In June 2012, the Chinese manned submersibleJiaolong was able to reach 7,020 m (23,030 ft) deep in the Mariana Trench, making it the deepest diving manned research submersible.^[33][34] This range surpasses that of the previous record holder, the Japanese-madeShinkai, whose maximum depth is 6,500 m (21,300 ft).^[35]

Few unmanned submersibles are capable of descending to maximum hadal depths. The deepest diving unmanned submersibles have included theKaikō (lost at sea in 2003),^[36] theABISMO,^[37] theNereus (lost at sea in 2014),^[28] and theHaidou-1.^[38]

• Abyssal plain – Flat area on the deep ocean floor
• Deep sea – Lowest layer in the ocean
• Deep submergence vehicle – Self-propelled deep-diving crewed submersible
• Abyssal zone – Deep layer of the ocean between 4000 and 9000 meters
• Sunlight zone – Uppermost layer of a sea water column that is exposed to sunlightPages displaying short descriptions of redirect targets; includes shallow waters and coral reefs

• Forscher filmen lebende Fische in Rekordtiefe (In German) from Spiegel 10/09/2008 about an expedition filming fish at a depth of more than 7,000 m
• Hadal highlights.Schmidt Ocean Institute. 11 January 2015.Archived from the original on 2021-12-21 – via YouTube.

• Hydrology
• Oceanographical terminology
• Extreme points of Earth
• 1950s neologisms

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