unfathomable

adjective

1 the dark unfathomable eyes of a stranger:

inscrutable, incomprehensible, enigmatic, incalculable, indecipherable, obscure, esoteric, abstruse, puzzling, cryptic, mysterious, mystifying, baffling, deep, profound, secretive.

▷antonyms comprehensible, penetrable.

2 a pool of dark, unfathomable water:

deep, immeasurable, fathomless, unfathomed, unplumbed, unplumbable, bottomless, unsounded, profound.

▷antonyms shallow.

unfathomable Oxford Dictionary of English