Last week I visited the Lorient Keroman U-Boote Base. This place was chosen by Doenitz as early as 1940 to become the largest U-Boote facilitly on the Atlantic coast. It was taken over by the French Navy in 1945 and used until 1997 . Nowadays Keroman 3 is a museum whereas K1 and K2 are used as warehouses. from 1941 three huge bunkers shelters were built by the Todt organisation, each new bunker coming with new techniques to counter the allied bombings. 1) Keroman 1 was built first and could hosted the first generation u-Boote notice the original cammo from 1944 2) Keroman 2 was built next to K1 and hoted bigger subs with better protection. It was connected to K1 but was separated by a 90meter open track that was still vulnerable. 3)K3 was built later with better technology (no open space and 7 meters thick concrete protection instead of three to which was added blue granite imported from Norway . 4) a K4 was planned in 1943 , but never completed . This one even planned a protected railroad station. see open hall on pic below.