Fathom

1 .to discover the meaning of something:
For years people have been trying to fathom (out) the mysteries of the whale's song.
2 .to understand someone or why someone acts as they do:
I can't fathom her at all.

fathomless, adjective LITERARY (BE)
impossible to understand:
I'm afraid it's a fathomless mystery.

fathomable, adjective, capable of being readily understood (AE)

unfathomable, adjective
1. Difficult or impossible to understand; incomprehensible: unfathomable theories.
2. Difficult or impossible to measure: the unfathomable depths.