Abyssal

1. (archaic) Belonging to, or resembling, an abyss; unfathomable. [First attested in the mid 17^th century.]^[1]
2. (geography) Of or belonging to the ocean depths, especially below 2000 metres (6500 ft): abyssal zone. [First attested in the mid 19^th century.]^[1]
3. (geology) Pertaining to or occurring at excessive depths in the earth's crust; plutonic. [First attested in the late 19^th century.]^[1]