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.