abyssal

Short definition from Wiktionary

Adjective

1. 1
   Belonging to, or resembling, an abyss; unfathomable.
2. 2
   Of or belonging to the ocean depths, especially below 2000 metres (6500 ft): abyssal zone.
3. 3
   Pertaining to or occurring at excessive depths in the earth's crust; plutonic.