unfathomable

adjective

1. not able to be fathomed, or completely understood; incomprehensible: heroism in the face of unfathomable conflict.
2. not able to be measured with a sounding line, or fathomed: unfathomable depths of the ocean.

adjective

1. incapable of being fathomed; immeasurable
2. incomprehensible

adj.1610s, originally in the figurative sense, of feelings, conditions, etc., from un- (1) “not” + fathomable. Literal sense attested from 1670s. Related: Unfathomably.