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Adjective: unfathomable  ,ún'fa-dhu-mu-bul
  1. Resembling an abyss in depth; so deep as to be unmeasurable
    "the unfathomable depths of the ocean";
    - abysmal [literary], abyssal
  2. Incapable of being fully understood
  3. Of depth; not capable of being sounded or measured

See also: deep, incomprehensible, uncomprehensible, unsoundable

Antonym: fathomable