1. Incapable of being fully understood
   "The reasons for his sudden departure remained unfathomable"
    
2. Of depth; not capable of being sounded or measured
   "The unfathomable ocean depths held many mysteries"
    
3. Resembling an abyss in depth; so deep as to be unmeasurable
   "the unfathomable depths of the ocean";
   - abysmal [literary], abyssal

See also: deep, incomprehensible, uncomprehensible, unsoundable

Antonym: fathomable

Encyclopedia: Unfathomable