Early in 1940 the Germans realized the vulnerability of their open top U-Boat enclosures and decided to start building massive bunkers to guard their precious assets from air attack. One of the first bunkers they build was the Elbe II, located near Hamburg.
Later when the French Atlantic coast was captured, submarine pens were built there too, They were built to last, as most are still intact and survive to this day, demolishing them would simply be too expensive.
Here are a selection of submarine pen pictures from WWII and beyond.
St. Nazaire.
Brest.
Inside a bunker, France. Location unkown.
St. Nazaire. 
Lorient, under construction.
Lorient.
U-67 going into Lorient Bunker
Unidentified location in France.
Construction in Bremen.
Buchiems U-Boat Pens
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
Under cover safe from bombs repair work goes on.
St.Nazaire under construction
Almost home – in 1943/44 the RAF fought a hard and protracted series of air battles across the approaches to the Biscay ports , safe in port the U-boat was often killed on the way in and out of their home ports.
From mid 1943 onwards the waters in the pens would be the only waters on which the U-boat could feel safe.
U-boat pens at Ijmuiden
Some of the bunkers as they are today
Submarine pen at Bordeaux
Roof of the U-boat base in Saint Nazaire.
Entrance to submarine pen on Croatia.
Keroman I and Keroman III, Lorient
