The Collaborative International Dictionary of English v.0.48:

Impenetrableness \Im*pen"e*tra*ble*ness\, n.
   The quality of being impenetrable; impenetrability.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impenetrableness
    n 1: incomprehensibility by virtue of being too dense to
         understand [syn: impenetrability, impenetrableness]