Submarine pen A submarine pen ( " U - Boot - Bunker " in German ) is a bunker which is designed to protect submarines from air attack . The term is generally applied to submarine bases constructed during World War II , particularly in Germany and the occupied countries which were also known as U - boat pens ( after the phrase " U - boat " to refer to German submarines ) . German submarine pens in World War II . Background . Amongst the first forms of protection for submarines were some open - sided shelters with partial wooden foundations that were constructed during the first World War . 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 . An RAF raid on the capital in 1940 plus the occupation of France and Great Britain ' s refusal to surrender was enough to trigger 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 " , 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 . The incessant air raids caused serious disruption to the project , hampering the supply of material , destroying machinery and harassing the workers . Machinery such as excavators , piledrivers , cranes , floodlighting and concrete pumps ( which were still a relatively new technology in the 1940s ) was temperamental , and in the case of steam - driven equipment , very noisy . Bunkers had to be able to accommodate more than just U - boats ; space had to be found for offices , medical facilities , communications , lavatories , generators , ventilators , anti - aircraft guns , accommodation for key personnel such as crew - men , workshops , water purification plants , electrical equipment and radio testing facilities . Storage space for spares , explosives , ammunition and oil was also required . Types of bunker . Four types of bunker were constructed : These were bunkers built over an existing lock to give a U - boat some protection while it was at its most vulnerable - i . e . when the lock was emptying or filling . They were usually constructed with new locks alongside an existing structure . Used for building new boats After launch , many U - boats were fitted - out under their protection This was the most numerous type . There were two types that were built either on dry land or over the water . The former meant that U - boats had to be moved on ramps ; the latter enabled the boats to come and go at will . Pumping the water out enabled dry dock repairs to be carried out . Some bunkers were large enough to allow the removal of periscopes and aerials . There is no truth in the rumour of an underground bunker on Fuerteventura in the Canary Islands . This ' story ' was gleaned from a similar situation in Le Havre in France when captured U - boat men were interrogated by the British . Locations . Pens were constructed in the northern coastal ports of the " Reich " and in many occupied countries . Germany . Pens protecting construction of the Type XXI submarine were located in Hamburg ( Blohm & Voss ) , Bremen ( AG Weser ) , and Danzig ( F . Schichau ) . Bremen . The " Hornisse " bunker was not started until 1944 in Bremen ; it was never completed . " Valentin " was the largest bunker in Germany . Begun in 1943 , it was built to accommodate the Type XXI submarine construction programme . It too was never completed . Post - war , the area was used as a test site for new bombs . Most of the damage done to the bunker was inflicted at this time . Danzig . Being out of range of Allied aircraft , no pens were built in Danzig ( now Gdansk in Poland ) . Hamburg . The city was the site of two structures , " Elbe II " and " Fink II " . The Finkenwerder bunker was constructed by 1 , 700 slave labourers over four years . After capture , it was demolished with 32 tonnes of bombs . Helgoland . The " Nordsee III " bunker was one of the oldest , being started in 1940 . It was left alone until near the end of the war when it was attacked by the RAF . It was also used after the end of the war for testing new weapons . No trace of the pen has survived . Kiel . This town was constantly bombed in World War II , the targets often being the " Kilian " and " Konrad " bunkers . They were started in 1941 and 1942 respectively . The latter was used for the construction of " Seehund " midget submarines . It was in " Kilian " that " U - 4708 " was probably the only submarine to be lost in a bunker . Misguided bombs from an air raid on the town caused what might today be called a " tsunami " to cross the " Förde " and enter the bunker . " Oberleutenant zur See " Hans - Gerold Hauber , the captain of " U - 170 " , had courted ridicule by ordering all hatches on his boat to be closed , despite being in the bunker . " This simple precaution saved " U - 170 " from sinking while lying next to " U - 4708 " " . Wilhelmshaven . A U - boat bunker in Wilhelmshaven was planned but never got beyond the preliminary stage . France . The German occupying forces built many U - boat pens in the Atlantic ports of France in Bordeaux , Brest , La Rochelle / La Pallice , Lorient and Saint - Nazaire . Almost 4 . 4 million cubic metres of concrete was used . Bordeaux . An unnamed bunker and bunkered lock were constructed in Bordeaux , the fourth largest French city at the start of the war . Both structures were started in 1941 ; the bunkered lock was not finished by war ' s end . The main building was larger than those in other locations ; this was to allow supply boats and minelayers to use it . The Italian Navy established the Betasom base at Bordeaux . The port was also the target of a British commando raid - the so - called Cockleshell Heroes . Brest . The Brittany port only had one bunker , but it was the largest ; it was also unnamed . Started in 1941 , the plans were modified many times before completion a year later . By February 1942 the RAF had lost interest in the area ; most of the town had already been destroyed and they did not possess large enough bombs to seriously threaten the bunker . Between February 1942 and early 1943 , apart from a few American aircraft , the place was left alone . The German garrison surrendered to US forces in September 1944 . They had had sufficient explosives to cripple the bunker but did not use them due to the proximity of a hospital . La Rochelle / La Pallice . Only six kilometres separate La Rochelle and La Pallice so they are usually considered as one port . An unnamed bunker was built at La Pallice ; it was started in April 1941 . Similar building techniques to those used in St . Nazaire were employed . Due to the relative ease of construction , the main structure was ready for its first U - boats six months later . A bunkered lock was begun in June 1942 . It was completed in March 1944 . Scenes for the film " Das Boot " ( 1981 ) were shot in La Pallice . Lorient . The largest U - boat base was in Lorient . Three bunkers , " Keroman I " , " II " and " III " , the " Scorff " bunker and two " Dom " bunkers , east and west , were all begun in 1941 . Two more were in the planning stage . " Keroman I " was unique in that it required its U - boats to be " hauled out of the water , placed on a many - wheeled buggy and then transported into the bunker on a sliding bridge system . " This arrangement might have been more vulnerable to air raids , but damage was minimal and it had the advantage of the U - boat not needing a dry dock . " Keroman II " , being landlocked , was served by the same system . " Keroman III " was more conventional , as was the " Scorff " bunker . The two " Dom " bunkers , ( so - called because of their resemblance to the religious building , " Dom " means ' cathedral ' in German ) , were located around a massive turntable which fed U - boats into the covered repair bays . Karl Dönitz , head of the U - boat arm and later the chief of the German navy , had his headquarters at nearby Kernevel . St - Nazaire . Construction of a no - name bunker was commenced in 1941 , as was a bunkered lock . ( But it should be noted that elsewhere in the reference , it states that " the excavations " for the bunkered lock were begun in October 1942 ) . The pens were not affected by the British commando raid in March 1942 , whose main objective were the " Normandie " dock gates . Norway . Norway is to some extent ruled by its weather . Building submarine pens was often hampered by snow and ice ; the ground might have been chosen , but the occupation of France only a few months after Norway ' s surrender rather overshadowed the Scandinavian country as far as bunkers for U - boats was concerned . Nonetheless , a requirement for protection was identified . With the liberation of France in 1944 , Norway regained its importance , but for barely a year . The Norwegian bunkers in Bergen and Trondheim were originally designed to have two floors , the lower one for U - boats , the upper one for accommodation , workshops and offices . However , with the project running six months late , plans for the second storey were abandoned . Bergen . Control of the Bergen project came under the German Naval Dockyard . Construction of " Bruno " commenced in 1941 , with a Munich - based firm taking the lead . A shortage of labour was , along with the acquisition of raw materials in sufficient quantities and poor weather was always going to cause problems . Specialised machinery had to be imported , as did accommodation that could stand up to the harsh Norwegian winter . In a bid to increase its protection , the bunker had granite blocks , each about a cubic metre in size , positioned on its roof . The shortage of cement ensured that the blocks could not be properly stuck down . Trondheim . Work on " Dora II " started in 1942 . It was not completed . " Dora 1 " had started the previous year , shortly after Operation Barbarossa , the invasion of the Soviet Union . This was fortuitous , as a ready supply of Russian prisoners of war ( POW ) s ( all volunteers ) , became available . Despite any number of precautions being taken when putting in the foundations , Dora I developed a noticeable sag of . It did not seem to bother the submariners as much as the builders . The Allied bombing offensive . U - boat facilities first became a bombing priority in March 1941 and again during the Combined Bomber Offensive . The bunkers did not suffer as much as their surroundings until August 1944 when a new type of bomb was used against them , the " Tallboy " . U - boat yards and pens were the primary objectives for the US Eighth Air Force from late 1942 to early 1943 . In the course of the war , the Allies used Operation Aphrodite radio - controlled aircraft , " Bat " guided bombs , " Disney " rocket - assisted bombs , Tallboy and Grand Slam deep penetration bombs to attack the U - boat pens . A U boat pen concrete target had been built in the at Ashley Walk bombing range in the New Forest , Hampshire , to assist in preparation for these raids . It consisted of a concrete roof covering three shallow " pens " . After the war it was buried in an earth mound , although its edges are once again visible in places due to weathering . Croatia . Jugoslav People ' s Army used submarines as well , one of the pens is located on island of Vis ; it is carved inside natural hill and is now abandoned and freely accessible from sea or by foot . Location :