“+ adjective
Etymology: un- (I) + fathom + -able
: not capable of being fathomed
unfathomable depths
a. : incomprehensible , inscrutable
the unfathomable mentality of a child
all that is cryptic and unfathomable in humanity — J.L.Lowes
b. : immeasurable , impenetrable
the unfathomable darkness of empty space — H.G.Wells
• un·fath·om·able·ness noun -es