Administration backers acclaim the Drug Wars concept as a foolproof and impregnable barrier against drug trafficking from abroad.

Long a separate culture when the mountains formed an almost impregnable barrier, this Berber region recalls the sweep of the American Southwest.

From these globes the atomic energy would flame out, to encircle the planet and transform the atomic structure of the Heaviside Layer into an impregnable barrier.

Gomez reflexively checked her shoulder harness as Corsi and Stevens flew the Kwolek toward that apparently impregnable barrier at a steeply decelerating rate.

It needed but a moment's effort to convince him of the futility of endeavoring to force that impregnable barrier.

Between me and Zara there seemed to be an invisible barrier, strong, and absolutely impregnable.

From here it looked a truly formidable and impregnable barrier.

His back was to the river, believing that to be an impregnable barrier to attack.

"Now we are on either side of an impregnable barrier."

An impregnable wooden barrier nailed across the door into the walls from the outside.