11.   Tunneling is an astonishing but common process whereby a particle can approach an apparently impenetrable barrier, disappear, and reappear on the other side.

12.   Tucker wants impenetrable barriers between airline cockpits and cabins, and he favors allowing pilots to carry guns.

13.   When a child gets defensive about being defensive, he creates an almost impenetrable barrier.

14.   The Alps long posed an almost impenetrable barrier to travelers and were a source of legends and superstitions.