Abyssal a. : Belonging to, or resembling, an abyss; unfathomable.

Example:
How is the Abyssal plain formed?

Taxonomy
Abyssal zone, 04-27, layers, free encyclopedia, abyssal, ocean depths, pelagic zone, the deep, trenches, search, Epipelagic, Aquatic ecology, abyssopelagic, Abyss, word meaning, bottomless sea, perpetual darkness, daylight.

Defination from the web:
abyssal zone is the abyssopelagic layer of pelagic zone that contains the very deep benthic communities near the bottom of oceans. ... The abyssal zone has temperatures around 2 to 3 degrees Celsius through the large majority of its mass.[2] ...