Synonyms for Unfathomable (same or very similar meaning)
WordNet sense 1 (so deep as to be unmeasurable):
abysmal, abyssalWordNet sense 2 (having great spatial extension or penetration downward or inward from an outer surface or backward or laterally or outward from a center; sometimes used in combination):
deepWordNet sense 3 (difficult to understand):
incomprehensibleFrom the ODE community, based on WordNetadd/edit