abyssal
/əˈbɪs.l̩/adj
University
1
Belonging to, or resembling, an abyss; unfathomable.
2
Of or belonging to the ocean depths, especially below 2000 metres (6500 ft): abyssal zone.
3
Pertaining to or occurring at excessive depths in the earth's crust; plutonic.
4
In the utter depths of despair; gloomy.
#archaic