Who first proposed the term hadal in 1956?

Anton Frederik Bruun first proposed the term hadal in 1956.

Bruun was a Danish zoologist and oceanographer who helped lead the Galathea Expedition, a major Danish deep-sea voyage conducted from 1950 to 1952. His work on deep-ocean life helped establish that the deepest trenches supported distinctive biological communities rather than simply being an extension of the abyssal seafloor.

He proposed hadal for ocean depths below about 6,000 metres, reserving abyssal for the zone from roughly 4,000 to 6,000 metres. The word refers to Hades, the ancient Greek underworld, a fitting allusion to the extreme darkness, pressure and isolation of oceanic trenches.

Scientists still commonly use 6,000 metres as the practical boundary, although some research suggests that the transition between abyssal and hadal conditions is gradual and that 6,500 metres may be a useful alternative threshold. About 94% of hadal environments occur in subduction trenches, including the Mariana, Tonga and Kermadec systems.