Hadal zone

The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. It ranges from about 6 to 11 kilometers below sea level, in long, narrow, V-shaped depressions.

Though it covers only a tiny part—less than 0.25%—of the world's seafloor, the hadal zone holds over 40% of the ocean's depth range. Most of these deep habitats are found in the Pacific Ocean, the largest and deepest of all the ocean areas.

Terminology and definition

The hadal zone is the deepest part of the ocean. People used to think it was just part of the abyssal zone, but in the early 1950s, scientists on the Danish Galathea II and Soviet Vityaz expeditions found that life changes a lot at depths of 6,000–7,000 m (20,000–23,000 ft). In 1956, Anton Frederik Bruun suggested calling this deepest part the "hadal" zone, named after Hades, the ancient Greek god of the underworld. Most of the hadal zone is found in deep subduction trenches.

Depths deeper than 6,000 m (20,000 ft) are usually in oceanic trenches, but some trenches are not deep enough to be hadal. While many people still use 6,000 m as the limit, some scientists suggest using 6,500 m instead. The animals in the hadal zone can be split into two groups: those that live on or near the bottom (hadobenthic) and those that swim in the water above (hadopelagic).

Ecology

The deepest parts of the ocean, called trenches, are some of the least explored places on Earth. These areas have no sunlight, very cold temperatures, and very high pressure. Most of the animals there feed on bits of dead material falling from above. Scientists have found over 400 different kinds of animals living in these deep trenches. Some of these animals are much bigger than we would expect, while others are quite small.

Even though it’s very deep, many types of animals still live there, like fish, sea cucumbers, and tiny worms. These animals have special ways to survive the high pressure and cold. Only a few kinds of fish live in the deepest parts, but some bigger animals, like certain squids and worms, can be found even deeper.

Conditions

The hadal zone is the deepest part of the ocean, where special bacteria act as the main source of food. These bacteria can live by using hydrogen and methane from reactions between rocks and seawater, or hydrogen sulfide from cold seeps. Some of these bacteria live inside certain types of small ocean creatures.

This zone can go deeper than 6,000 meters (20,000 feet), with the deepest known point reaching 10,911 meters (35,797 feet). Because there is no light and the pressure is extremely high—over 1,100 times the normal air pressure—this area is very hard to explore.

Exploration

Exploring the hadal zone needs special tools that can handle very high pressure—up to a thousand atmospheres or more. Some simple tools have been used to learn a little about the basic life there. Both people inside and robots outside can help study these deep places. Robots can be controlled from ships or move on their own. Cameras and tools on these robots let scientists watch and collect samples of mud and living things. Sadly, some robots have broken apart under the huge pressure of the deep sea. The HROV Nereus is thought to have broken apart at a depth of 9,990 meters while exploring the Kermadec Trench in 2014.

Notable missions

In 1960, two explorers named Jacques Piccard and Don Walsh made the first journey to the deepest part of the ocean, called Challenger Deep, which is in the Mariana Trench. They used a special underwater vehicle called Trieste and reached a depth of 10,911 metres.

Later, in 2012, a filmmaker named James Cameron also went to the bottom of the Mariana Trench alone in a vehicle called Deepsea Challenger. He reached a depth of 10,908 metres, just a little less deep than Piccard and Walsh.

Also in 2012, a Chinese underwater vehicle named Jiaolong reached a depth of 7,020 metres in the Mariana Trench, making it one of the deepest research vehicles ever made.