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, 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.