Keroman Submarine Base was a German U-boat base located in Lorient during World War II. Großadmiral Karl Dönitz decided to build the base on 28 June 1940. Between February 1941 and January 1942 three gigantic reinforced concrete structures were built on the Keroman peninsula. They are called K1, K2 and K3. In 1944 work began on a fourth structure. The base was capable of sheltering thirty submarines under cover. Although Lorient was heavily damaged by Allied bombing raids, this naval base survived through to the end of the war. Lorient was held until May 1945 by the Germans’ Wehrmacht Heer regular army forces, though surrounded by the American Army; the Germans refused to surrender.Since they could not destroy the base and its submarine pens, the Allies had decided to flatten the city and port of Lorient to cut the supply lines to the U-boat bases. Without resupply of fuel, weapons (e.g. torpedoes), and provisions, it became impossible for those U-boats to return to war patrols in the Atlantic Ocean. Between 14 January 1943 and 17 February 1943, Allied aircraft dropped as many as 500 high-explosive bombs and more than 60,000 incendiary bombs on Lorient; nearly 90% of the city was flattened.

After the war the base was used by the French Navy until 1997. Rechristened by the French as Base Ingénieur Général Stosskopf in July 1946, the new name commemorated Jacques Stosskopf, a German-speaking Alsatian Frenchman who had been the deputy director of naval construction for the Germans at the base while secretly in the French Resistance, and had given valuable information on submarine movements to the Allies during the war until his activities were discovered and he was killed.[1]

At present the Keroman U-boat base is open to the public. During tours, the submarine pens of block K3 can be seen. Its roof (3.40 to 7.0 metres (11.15 to 22.97 ft) of steel-reinforced concrete) can be visited, as well as a former anti-aircraft tower on top of the U-boat base. The tower affords an excellent view of the harbour and of the former headquarters of Großadmiral Karl Dönitz of the Kriegsmarine (Nazi German navy) across the bay at Larmor-Plage. The FloreS645, a Daphné-class submarine, launched 1961, can also be visited. Another part of the base has been reconverted for industrial naval activities, with the preparation of racing multihulls.

Source: Wikipedia