\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.