A submarine pen (U-Boot-Bunker in German) is a type of submarine base that acts as a bunker to protect submarines from air attack. The term is generally applied to submarine bases constructed during World War II, particularly in Germany and its occupied countries, which were also known as U-boat pens (after the phrase "U-boat" to refer to German submarines). Background Among the first forms of protection for submarines were some open-sided shelters with partial wooden foundations that were constructed during World War I. These structures were built at the time when bombs were light enough to be dropped by hand from the cockpit. By the 1940s, the quality of aerial weapons and the means to deliver them had improved markedly. The mid-1930s saw the Naval Construction Office in Berlin give the problem serious thought. Various factions in the navy were convinced protection for the expanding U-boat arm was required. A Royal Air Force (RAF) raid on the capital in 1940, the occupation of France and Britain's refusal to surrender triggered a massive building programme of submarine pens and air raid shelters. By the autumn of 1940, construction of the "Elbe II" bunker in Hamburg and "Nordsee III" on the island of Heligoland was under way. Others swiftly followed. General It was soon realised that such a massive project was beyond the Kriegsmarine, and the Todt Organisation (OT) was brought in to oversee the administration of labour. The local supply of such items as sand, aggregate, cement, and timber was often a cause for concern. The steel required was mostly imported from Germany. The attitudes of the people in France and Norway were significantly different. In France there was generally no problem with the recruitment of men and the procurement of machinery and raw materials. It was a different story in Norway. There, the local population were far more reluctant to help the Germans. Indeed, most labour had to be brought in. The ground selected for bunker construction was no help either: usually being at the head of a fjord, the foundations and footings had to be hewn out of granite. Several metres of silt also had to be overcome. Many of the workers needed were forced labour, most especially the concentration camp inmates supplied by the Schutzstaffel from camps near the pens. The incessant air raids caused serious disruption to the project, hampering the supply of material, destroying machinery, and