The Collaborative International Dictionary of English v.0.48:

Unfathomable \Unfathomable\
   See fathomable.

WordNet (r) 3.0 (2006):

unfathomable
    adj 1: of depth; not capable of being sounded or measured [ant:
           fathomable, plumbable, soundable]
    2: resembling an abyss in depth; so deep as to be unmeasurable;
       "the abyssal depths of the ocean" [syn: abysmal, abyssal,
       unfathomable]
    3: impossible to come to understand

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "unfathomable":
   Greek to one, abysmal, abyssal, all-comprehensive, all-inclusive,
   ambiguous, beyond understanding, bottomless, boundless, cavernous,
   countless, deep as hell, endless, eternal, exhaustless,
   extending everywhere, fathomless, gaping, illimitable, illimited,
   immeasurable, immense, impenetrable, inarticulate, incalculable,
   incognizable, incoherent, incomprehensible, inexhaustible,
   infinite, infinitely continuous, innumerable, inscrutable,
   interminable, interminate, limitless, measureless, no end of,
   numinous, past comprehension, perpetual, plumbless, plunging,
   rambling, shoreless, soundless, sumless, termless, unbounded,
   uncircumscribed, unconnected, unfathomed, ungraspable,
   unintelligible, universal, unknowable, unlimited, unmeasurable,
   unmeasured, unnumbered, unplumbed, unsearchable, unsounded, untold,
   without bottom, without bound, without end, without limit,
   without measure, without number, yawning