Etymology
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abyssal (adj.)

1690s, "unfathomable, unsearchably deep, like an abyss," from abyss + -al (1). Since 19c. mainly "inhabiting or belonging to the depths of the ocean" (used especially of the zone of ocean water below 300 fathoms), though in 19c. abysmal was more common in oceanography.

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Definitions of abyssal

abyssal (adj.)
relating to ocean depths from 2000 to 5000 meters;
abyssal (adj.)
resembling an abyss in depth; so deep as to be unmeasurable;
the abyssal depths of the ocean
Synonyms: abysmal / unfathomable
From wordnet.princeton.edu