1) Abysmal 2) Abyssal 3) Bottomless 4) Deep 5) Enigmatic 6) Inexplicable 7) Mind-blowing 8) Mysterious 9) Opaque 10) Uncomprehensible 11) Unintelligible 12) Unsearchable 13) Unsoundable 1) Deep 2) Of depth 3) Resembling an abyss in depth 4) So deep as to be unmeasurable 5) Unsoundable 
[adj] - of depth 2. [adj] - of meaning
adjective impossible to come to understand
[SAT terms] resembling an abyss in depth; so deep as to be immeasurable