unfathomable

Adjective (3)

1. of depth; not capable of being sounded or measuredOpposite fathomableSimilar unsoundableSee also deep
2. resembling an abyss in depth; so deep as to be unmeasurable“the abyssal depths of the ocean”Synonyms: abysmalabyssalOpposite shallowSimilar deep
3. incapable of being fully understoodOpposite comprehendibleSimilar incomprehensible, uncomprehensible