abyssal

adj

1. (archaic, not-comparable)Belonging to, or resembling, an abyss; unfathomable.
2. (not-comparable)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.”

   “"Ea", the beneficent god of earth and life, who dwelt in the abyssal waters, was shown with two streams running from his hands or shoulders[.]”

3. (not-comparable)Pertaining to or occurring at excessive depths in the earth's crust; plutonic.
4. (figuratively, not-comparable)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.”