hadal: Referring to the part of the ocean below 6000 meters, or approximately 3.73 miles.
Hadal (rhymes with cradle) is an adjectival form of the word Hades, which makes a certain amount of sense, though I never would have thought of using the word in such an oceanographic way.
That 6000-meter line marks the beginning of the hadalpelagic zone, the deepest parts of the ocean. Pelagic means about or occurring in the open sea, so hadalpelagic isn't much more descriptive than just hadal.
For reference and for fun, here are the "layers" of Earth's oceans:
Epipelagic: 0–200 meters
Mesopelagic: 200–1000 meters
Bathypelagic: 1000–2000 meters
Abyssapelagic: 2000–6000 meters
Hadalpelagic: deeper than 6000 meters