unfathomable

 
Pronunciation: /ʌnˈfað(ə)məb(ə)l/

adjective

  • 1incapable of being fully explored or understood: her grey eyes were dark with some unfathomable emotion
  • 2(of water or a natural feature) impossible to measure the extent of: mountains of unfathomable scale

Derivatives

unfathomably

adverb