unfathomable vs abyssal

unfathomable and abyssal both are adjectives.

Word	Noun	Adjective	Verb	Adverb
unfathomable	No	Yes	No	No
abyssal	No	Yes	No	No

As adjectives, unfathomable and abyssal are synonyms defined as:

unfathomable and abyssal: resembling an abyss in depth; so deep as to be unmeasurable

Other synonyms of unfathomable include abysmal.

unfathomable (adjective)	abyssal (adjective)
of depth; not capable of being sounded or measured	relating to ocean depths from 2000 to 5000 meters
resembling an abyss in depth; so deep as to be unmeasurable	resembling an abyss in depth; so deep as to be unmeasurable
incapable of being fully understood