A scene from "U-Boat Pens"
To create a haven in port for their lethal U-boat submarines, the Nazis built massive, impenetrable concrete submarine pens. Structures too immense to be hidden, they were constructed to withstand direct hits from even the biggest Allied bombs. Such was their size and strength that these pens survive today, a testament to their engineering.

Nazi Mega Weapons: U-Boat Base, Wednesday at 10 p.m. on PBS 6.

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