From The Collaborative International Dictionary of English v.0.48:

Abyssal \A*byss"al\, a. [Cf. Abysmal.]
   Belonging to, or resembling, an abyss; unfathomable.
   [1913 Webster]

   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.
      [1913 Webster]
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