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.

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.

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.

Almost Indestructible

These submarine pens were more than just massive bunkers. They were also designed to provide office spaces, accommodation for personnel, medical facilities, storage of the supplies and areas of the submarines, ammunition, anti-aircraft defenses, fuel, and power generator.

As the main purpose of these pens was to provide protection, it was no surprise that they were meant to be immune from all sorts of Allied bombs, with roofs as thick as 26 feet: a width proved effective against enemy bombs. This was proved to be true when these U-boat facilities became a bombing priority in March 1941 and again in the Combined Bomber Offensive. From late 1942 to 1943, the Allies tried various tactics and approaches to take the pens down. For instance, as part of Operation Aphrodite, the US Army Air Forces used bat-guided bombs, while the RAF designed the “Disney” rocket-assisted bombs. Nevertheless, these thick bunkers stood tall and unchanged compared to their surrounding areas severely damaged by the bombings.

As written by Air Force Magazine,

The effort cost the Eighth 135 bombers (including 16 written off), for an unsustainable 5.9 percent loss rate. Shockingly, almost 1,200 airmen were killed or captured in the 119 missing aircraft.

It was not until August 1944 that the Allied forces introduced a new type of bomb that could rival these tough pens, the earthquake bombs that British scientist, engineer, and inventor Sir Barnes Neville Wallis created: The Tallboys and Ten Ton Tess. These bombs were six and ten tons, respectively. They worked by burrowing on the ground beside the target building before exploding and causing an earthquake-like reaction, destroying the foundation of the buildings.