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.

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. Most hadal habitat is found in the Pacific Ocean, the deepest of the conventional oceanic divisions.

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. 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). The name refers to Hades, the ancient Greek god of the underworld. About 94% of the hadal zone is found in subduction trenches.

Depths in excess of 6,000 m (20,000 ft) are generally in ocean trenches, but there are also trenches at shallower depths. These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal. 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, 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 by UNESCO. 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.