The Collaborative International Dictionary of English v.0.48:

Fathomable \Fath"om*a*ble\, a.
   Capable of being fathomed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fathomable
    adj 1: (of depth) capable of being sounded or measured for depth
           [syn: fathomable, plumbable, soundable] [ant:
           unfathomable]
    2: (of meaning) capable of being penetrated or comprehended

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "fathomable":
   appraisable, appreciable, apprehensible, articulate, assessable,
   calculable, cognizable, comprehensible, computable, determinable,
   easily understood, easy to understand, estimable, exoteric,
   gaugeable, graspable, intelligible, knowable, lucid, luminous,
   measurable, mensurable, meterable, noticeable, numerable,
   penetrable, perceptible, plumbable, prehensible, quantifiable,
   quantizable, readable, scrutable, understandable