It was the Navy's largest construction project - the "Valentin" bunker. Up to 12,000 forced laborers toiled to build this colossus out of the ground on the Weser from 1943 to March 1945. Thousands also paid with their lives.

Accompany me on a tour of this “concrete monster”.

CORRECTION: the submarine was of course type "XXI", not "XI". Thanks for the tip in the comments!

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https://www.denkort-bunker-valentin.de