The Valentin submarine pens was a bunker for U-boats during World War II. The pens were built from 1943 to March 1945 but got never finished. The Valentin U-boat pens were the largest fortified pens in Germany, and were second only to those built at Brest in France.
A bomb proof shelter was built because of the bombing of the RAF and the USAAF. These bombings were succesfully trying to reduce the production of U-boats, until the bomb-proof bunkers were built. The pens had ferrous concrete roofs up to 7 metres thick.