Hadal zone

The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. 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.^[3] Most hadal habitat is found in the Pacific Ocean, the deepest of the conventional oceanic divisions.^[3]

Terminology and definition

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 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.^[4][5] The term "hadal" was first proposed in 1956 by Anton 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 to Hades, the ancient Greek god of the underworld.^[6] About 94% of the hadal zone is found in subduction trenches.^[7]

Depths in excess of 6,000 m (20,000 ft) are generally in

The deepest ocean trenches are considered the least explored and most extreme marine

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.[25] Manned and unmanned submersibles, however, can be used to study the depths in greater detail. Unmanned robotic submersibles may be remotely operated (connected to the research vessel by a cable) or autonomous (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 the Kermadec Trench in 2014.^[26]

Notable missions

The first manned exploration to reach

James Cameron also reached the bottom of Mariana Trench in March 2012 using the Deepsea Challenger.^[29] 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.^[30] Cameron holds the record for the deepest solo dive.^[28]

In June 2012, the Chinese manned submersible Jiaolong was able to reach 7,020 m (23,030 ft) deep in the Mariana Trench, making it the deepest diving manned research submersible.^[31][32] This range surpasses that of the previous record holder, the Japanese-made Shinkai, whose maximum depth is 6,500 m (21,300 ft).^[33]

Few unmanned submersibles are capable of descending to maximum hadal depths. The deepest diving unmanned submersibles have included the Kaikō (lost at sea in 2003),^[34] the ABISMO,^[35] the Nereus (lost at sea in 2014),^[26] and the Haidou-1.^[36]

See also

• 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
   – The uppermost layer of a sea water column that is exposed to sunlightPages displaying short descriptions of redirect targets – A layer that includes shallow waters and coral reefs

References