The hadal zone (named after the realm of Hades, the underworld in Greek mythology), also known as the hadopelagic zone and composed of trench zones, is the delineation for the deepest trenches in the ocean. The hadal zone is found from a depth to the bottom of the ocean of around 6,000 to 11,000 metres (20,000 to 36,000 ft) and exists in long but narrow topographic V-shaped depressions.
"In total there are 33 trenches (27 subduction trenches and 6 trench faults) and 13 troughs around the world—46 individual hadal habitats in total." All the trenches together occupy an area less than one-quarter of one percent of the entire seafloor, with 84% of the hadal habitat found in the Pacific Ocean.
The hadal zone has low population and low diversity of marine life.
Hadal zone Wikipedia
It is believed that most life at this depth is sustained by marine snow or the chemical reactions around thermal vents. The low nutrient level, extreme pressure and lack of sunlight create hostile living conditions in which few species are able to exist. As no sunlight reaches this layer of the ocean, deep sea creatures have reduced eyesight, with very large eyes for receiving only bioluminescent flashes.
The most common organisms include jellyfish, viperfish, tube worms and sea cucumbers. The hadal zone can reach far below 6,000 meters (20,000 feet) deep; the deepest known extends to 10,911 meters (35,814 ft). At such depths (for example, at 36,000 feet below sea level) the pressure in the hadal zone exceeds 1,100 standard atmospheres (110 MPa; 16,000 psi).
In 1960, Jacques Piccard and Don Walsh reached the bottom of the Mariana Trench, the deepest known trench on Earth, and observed life. James Cameron also reached the bottom in 2012 using the Deepsea Challenger.