The name hadal refers to which Greek god?

The name hadal refers to Hades, the ancient Greek god of the underworld.

Scientists use “hadal” for the deepest parts of the ocean, usually the regions found below about 6,000 metres in deep-sea trenches. The term was proposed in 1956 by Danish oceanographer Anton Frederik Bruun, after expeditions found that life in these extreme depths differed from life in the abyssal zone above.

The reference is mythological: Hades ruled the Greek underworld, a hidden realm associated with darkness and great depth. The name is not derived from Poseidon, the god of the sea, although Poseidon is the Greek deity most directly associated with oceans.

Hadal environments have no sunlight, near-freezing temperatures and pressures exceeding 1,000 times atmospheric pressure. Most known hadal habitats are in tectonic trenches, where one oceanic plate is forced beneath another. Challenger Deep, in the Mariana Trench, is the deepest known example.