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).

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.

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.

Four types of bunker were constructed:

• Covered lock.- 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.
• Construction bunker.- Used for building new boats
• Fitting-out bunkers.- After launch, many U-boats were fitted-out under their protection
• Shelter for operational boats and repair bunkers.- 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.... (see more at: http://en.wikipedia.org/)

Nazi Mega Weapons

About The series.- In a quest for world domination, the Nazis built some of the biggest and deadliest pieces of military hardware and malevolent technology in history. The six-part series Nazi Mega Weapons recounts Germany’s engagement in World War II from a unique perspective, uncovering the engineering secrets of iconic megastructures, telling the stories of the engineers who designed them and revealing how these structures sparked a technological revolution that changed warfare forever.

With historic footage, recreations of milestone moments and interviews with history, engineering, and military strategy experts, Nazi Mega Weapons recounts the strategic triumphs and wartime blunders of the Third Reich.

“The scope of these projects and the superhuman efforts that brought them to fruition remains astonishing, even 70 years later,” said Executive Producer Simon Young, “We want viewers to understand the extraordinary lengths to which the Nazis went in order to realize their ill-fated dream of world domination. This was a fascinating era of history, made even more chilling by the fact that some many of these structures are still around today.” 

About U-Boat Base.- To create a haven in port for their lethal U-boat submarines, the Nazis built massive, impenetrable concrete submarine pens. Structures too immense to be hidden, they were constructed to withstand direct hits from even the biggest Allied bombs. Such was their size and strength that these pens survive today, a testament to their engineering.