abyssal
This definition is taken from a public domain 1913 edition of Webster's Dictionary: please see Abyssal for the new Wiktionary definition of this term.
Abyssal, adjective
[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.