These were adequate when bombs were small enough to be manually dropped from aircraft cockpits. However, by the 1940s, advancements in aerial weaponry and delivery systems had significantly evolved.

In the mid-1930s, the Berlin Naval Construction Office began seriously considering solutions for protecting the growing submarine fleet.

Different segments within the navy agreed on the need for such defenses. The escalation of threats, including a Royal Air Force (RAF) attack on the German capital in 1940, the occupation of France, and Britain’s steadfast resistance, spurred an extensive construction effort for submarine pens and air raid shelters.

By the fall of 1940, the building of major submarine bunkers, including “Elbe II” in Hamburg and “Nordsee III” on Heligoland, had commenced, with more quickly following suit.

New U-Boat bunker

In 1940, plans were approved for a new U-Boat bunker at the Howaldtswerke shipyards along the River Elbe in Hamburg, with construction starting towards the end of the year.

The project, undertaken by Dyckerhoff & Widmann AG, was situated at the eastern end of the Vulkan basin.

By March 1941, the bunker was completed, featuring two pens each measuring 112 meters long and 22.5 meters wide, with enough space to house three submarines side by side.

U-3506, U-3004 and U-250S were scuttled inside the pen

Behind the bunker at ground level, the western section was allocated for storage, while the eastern side housed offices and electrical equipment.

The entire upper floor of this rear section was equipped with workbenches and machinery such as drills, vertical and horizontal boring machines, and lathes, facilitating on-site repairs and modifications.

Entry to the bunker was secured by small, heavily fortified steel doors on either side.

When British troops advanced on Hamburg in May 1945, the crews scuttled U-3004, U-2505 and U-3506 in the bunker.

Primarily, the Elbe II bunker served as a protective shelter for the new Type XXI U-Boats during their outfitting, as well as a repair and refit site for active submarines.

Despite repeated Allied bombing raids, the U-Boats inside the bunker remained largely unscathed, though the structure’s roof sustained some damage.

Large Bomber Raid

The most significant bombing assault occurred on the night of March 8, 1945, when over 300 RAF bombers targeted the Hamburg dock area, releasing nearly 1,000 tons of explosives.

This attack marked the beginning of a focused bombardment campaign against the dock facilities in Hamburg.

At the end of March, the RAF intensified their efforts, deploying over 450 bombers to drop more than 2,200 tons of bombs on the area, devastating the Howaldtswerke yards but causing minimal damage to the bunker itself.

The u-boats were then forgotten until 1985 when Jak P Mallmann-Showell, Wolfgang Hirschfeld, and Walter Cloots rediscovered the boats in what remained of the mostly destroyed Elbe II U-boat bunker.

However, during an air raid on April 8, 1945, involving another onslaught of over 400 bombers, the massive steel doors guarding the bunker’s entrance were blown off, although the core structure remained intact.

The initial groundwork, which began in December, relied on the caisson foundation technique, later also applied to the Bunker Fink II submarine.

This method was chosen due to Hamburg’s challenging subsoil of alluvial sand, where the caisson foundation offered considerable benefits for both Elbe II and later, Fink II.

The process involved using compressed air caissons, rectangular structures of sheet piling or concrete. Workers excavated the soil from a working chamber, open at the bottom and enclosed by a ceiling and outer walls, with compressed air displacing water from the chamber.

For Elbe II’s foundation, 2,500 piles, each measuring 38 x 35 cm with a load capacity of 60 tons, were driven into the harbor’s subsoil.

Roof Was Nine Feet Thick

All excavation and spatial construction work was completed by March 1941. The foundation, supported by these piles, included a two-floor workshop area at the back.

Bombproof office and administrative spaces were integrated into the building’s wings. To construct the walls, ceilings, and supports, Dyckerhoff & Widmann AG, guided by Prof. Dr. Agatz, and used approximately 50,000 m³ of reinforced concrete.

The bunker’s ceiling was 3.0 meters thick, and its outer walls were 2.5 meters thick. It had two bunkers, each equipped with a 3-ton overhead crane, measuring 112.5m x 22.5m, and connected to a railway and a 1,800 m² workshop area spread over two floors.

As World War II drew to a close, the Elbe II bunker sustained several hits. A Tallboy bomb strike in 1945 resulted in the 3-meter-thick ceiling curving slightly inward.

An attack on April 8, 1945, led to partial destruction of the hanging gate locks due to a nearby explosion. In the final days of the war, three submarines – U-3004, U-2505, and U-3506 – were deliberately sunk by their crews in western pens.

On the 11th of November 1945, Royal Engineers from the British Army detonated several well placed sets of explosives (mainly discarded Luftwaffe ordinances) in an attempt to implode the whole complex.

On November 11, 1945, the Royal Engineers executed a plan to demolish the bunker, using 32 tonnes of explosives. The resulting partial collapse of the roof effectively entombed the U-boats within the bunker.

In the late 1940s or early 1950s, efforts were made to salvage the trapped boats.

While some components were removed, the endeavor was deemed too hazardous, particularly because one of the U-boats was pinned beneath the collapsed roof.

Consequently, the salvage operation was quickly discontinued.

The British proceeded to blow up the bunker with 32 tonnes of bombs which caused the roof to partially collapse, trapping the u-boats in the bunker.The remains of the Elbe II bunker, photographed in 1981

The U-boats remained largely forgotten until 1985, when Jak P Mallmann-Showell, Wolfgang Hirschfeld, and Walter Cloots stumbled upon them in the remnants of the largely demolished Elbe II U-boat bunker.

Their exploration yielded stunning photographs, capturing the rediscovered U-boats.

Car Park

During the 1990s, the German government concluded that the bunker posed a significant hazard and took measures to neutralize this threat by filling it with gravel and concrete.

This action permanently entombed the U-boats. Subsequently, the site was transformed into a parking area and is now completely off-limits.

The Elbe II bunker is situated on the southern side of the Elbe River at Vulkanhafen, within Hamburg’s Freeport zone.

If you’re planning to visit this location, remember to carry your passport.

Were Germans U-boats found in the bunker?

Though the Germans set their bunker for an explosion, three U-boats inside remained. There was an attempt to access and scrap the submarines in the 1940s, but it was too dangerous.

Therefore, they remained in the bunker. However, in 1985, researchers rediscovered their location in the Elbe II bunker.