Valentin submarine pens

The Valentin submarine factory is a protective shelter on the Weser River at the Bremen suburb of Rekum, built to construct German U-boats during World War II. The factory was under construction from 1943 to March 1945 using forced labour, but was damaged by air-raids and unfinished by the end of the war. The Valentin factory was the largest fortified U-boat facility in Germany, and was second only to those built at Brest in France.

As a manufacturing facility, it differed from conventional U-boat pens, which were designed to house and service operational U-boats.

Source: Wikipedia