Abyssal \A*byss"al\, a. [Cf. Abysmal.]

Belonging to, or resembling, an abyss; unfathomable.

Abyssal zone (Phys. Geog.), one of the belts or zones into which Sir E. Forbes divides the bottom of the sea in describing its plants, animals, etc. It is the one furthest from the shore, embracing all beyond one hundred fathoms deep. Hence, abyssal animals, plants, etc.

Source: Webster's Revised Unabridged Dictionary (1913)