The Valentin submarine factory is a protective shelter built to construct German U-boats during World War II. The Valentin factory was the largest fortified U-boat facility in Germany.
Military - Bunkers
The Valentin factory was attacked by the RAF on March 27, 1945. On March 30 the US Eighth Air Force attacked. After the attacks the factory was abandoned.
Links: virtualglobetrotting.com, en.wikipedia.org