unfathomable

adjective3

1. •
   Impossible to fathom.
2. •
   Especially of depth: physically incapable of being measured; immeasurable.

   • Now, where the fiercest war among the waves / Is calm, on the unfathomable stream / The boat moved slowly. — 1815
3. •
   (figurative) Impossible to grasp the extent of, or to fully know or understand.

   • The sheer number of warriors the enemy attacked with was unfathomable.