abyssal vs unfathomable

abyssal and unfathomable both are adjectives.

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

As adjectives, abyssal and unfathomable are synonyms defined as:

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

Other synonyms of abyssal include abysmal.

abyssal (adjective)	unfathomable (adjective)
relating to ocean depths from 2000 to 5000 meters	of depth; not capable of being sounded or measured
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