abyssal

adjective4

1. •
   Of or belonging to the ocean depths, especially below 2000 metres (6500 ft): abyssal zone.

   • This "abyssal" life, typically in the form of strange tube-worms and microbes, is almost completely unknown to science. — 1994
2. •
   Pertaining to or occurring at excessive depths in the earth's crust; plutonic.
3. •
   (figurative) In the utter depths of despair; gloomy.

   • In an attempt to avoid sliding into incorrigible despair, I abandon such abyssal and existential pondering and investigate the artist behind the work. — 1998
4. •
   (no longer used) Belonging to, or resembling, an abyss; unfathomable.