3 sentences with “impenetrable”

Thom paddled on with all his might, ignoring the pain in his arms and back, but in that impenetrable blackness every minute felt like an eternity.

The world, Greek scientists thought, was not a sacred or impenetrable thing that could never be understood; they sought to explain it without recourse to supernatural forces.

The African continent was largely impenetrable to Europeans because of its geography: there were few ports for ships, the interior of the continent had no navigable rivers under sail and, most importantly, there were numerous lethal diseases (especially a particularly virulent form of malaria) to which Europeans had little resistance.