Not soundable; deep; profound; unfathomable.

adjective. Impossible to sound; whose depth cannot be gauged.

adjective. too deep to determine the depth of

"Before undertaking to fathom with the plummet of logic the unsoundable mystery of foreknowledge, let us see what we can know through a return to the real nature of man as he is, and especially to the real nature of the new Adam who is Christ, the Son of"

unfathomable