- unfathomable - of depth; not capable of being sounded or measured
Antonyms: fathomable, plumbable, soundable
- abysmal, abyssal, unfathomable - so deep as to be unmeasurable; "the abyssal depths of the ocean"
Antonym: shallow (indirect, via deep)
- unfathomable - impossible to come to understand
Antonyms: comprehensible, comprehendible (indirect, via incomprehensible)