During WWI and WWII, the Germans knew they can’t be complacent about defense, so they built what would become one of the war’s most famous concrete construction works and one of the most demanding targets in Europe: submarine pens. These impressive structures were constructed to protect the Nazi submarine fleet from aerial attacks during their vulnerable times of refueling, refit, or repairing, and they were almost indestructible.

The Need to Shelter the U-Boats

During World War I, submarines were initially protected through open-sided shelters made with partial wooden foundations. The seemingly crude shelters were built when the weight of bombs was light enough that soldiers could drop them by hand from the cockpit. By World War II, the means and quality of the aerial weapons greatly improved, and aircraft were now carrying combs weighing tons. So, the rickety shelters offer no protection from the improved bombs; they had to be significantly increased, too.

With the Royal Air Force (RAF) raid on Berlin in 1940 and the occupation of France and Great Britain’s refusal to surrender, various factions in the German navy were confident that they needed to protect their expanding submarine fleet.

Constructions Began

By the autumn of 1940, the construction of submarine bunkers began in La Rochelle’s harbor: the Elbe II in Hamburg and the “Nordsee III” on the Heligoland island, which was followed by others soon enough. The massive project made it clear from the beginning that it was beyond what the Kriegsmarine could manage to do, so the civil and military engineering establishment Organisation Todt was brought in to oversee the labor administration.

Construction of the U-boat base at La Pallice, 1942. (Bundesarchiv, Bild 101II-MW-6860-25A / Tölle (Tröller) / CC-BY-SA 3.0CC BY-SA 3.0 DE, via Wikimedia Commons)

Todt dealt with sand, aggregate, cement, and timber problems. Most of the steel was imported from Germany, and the local population was reluctant to help the Germans, so most workers had to be brought in, too. By that, we mean forced laborers from concentration camps near the construction area supplied by the Schutzstaffel. These enslaved laborers were subject to extreme conditions and inhumane treatment. With very little food and water supply, they often worked until they died from exhaustion and lack of food.

Former Uboat bunker of the German Kriegsmarine in Bordeaux, France, built-in 1942. (P. CharpiatCC BY-SA 2.5, via Wikimedia Commons)

The construction was not peaceful at all. There were usually air raids that caused disruptions to the project, shortage of materials, destruction of their machinery, and harassment of their workers. Also, at that time, the machines they used were pretty new technology, so their excavators, pile drivers, cranes, floodlighting, and concrete pumps were not as reliable.

As Atlas Obscura wrote in an article,

Because of their essential place in Nazi warfare, the U-boat pens had to be built in secret. Most of the construction was performed at night, illuminated by spotlight that would be extinguished at the hint of danger, leaving the workers (mostly Spanish and Portuguese prisoners of war) in pitch darkness.