When German forces took over France in mid-1940 the race began to protect their newly won territories with bunkers and batteries designed to repel any threats.
Alongside these were many sites designed to attack those threats, including V1 and V2 launch sites, each using massive amounts of concrete.
But all these sites are tiny in comparison to the massive constructions built to protect, service, and resupply the German Kriegsmarine’s U-Boat fleet – the most terrifying and effective weapons again the Allies at the time.
During the first year of the war over 1,000 Allied ships were sunk, more than half of these fell to the U-Boat menace which patrolled the Atlantic waters in search of targets.
Originally based in German ports such as Hamburg and Kiel, there was a need for U-Boats to be located closer to their main hunting grounds and so five sites were earmarked on the western French coast.
The five sites were based at exiting French ports at Brest, Lorient, Saint Nazaire, La Rochelle, and Bordeaux and they would become the largest structures built by the Nazis outside of Germany.
In July 1940 U-Boat U30 - built in Bremen and launched in August 1936 - had entered the harbour at Lorient, the first of many submarines which would operate from here over the next four years. At this point the U-Boats weren’t protected and so Fritz Todt, Hitler’s leading engineer, was tasked with building bunkers which would protect these vital assets.
Of the five sites, Lorient on the Keroman peninsula in Brittany was the first to be built and become operational. It would become the largest of them all with seven ‘cells’ built to protect up to 13 submarines at a time. The first three cells at Lorient were completed by June 30, 1941. Using a mixture of paid workers and slave labour, they took just three months to build.
Constructions at Brest and Saint Nazaire followed and by 1942 work was progressing at the sites at La Pallice near La Rochelle and the inland U-Boat bunker in Bordeaux on the River Garronne. The plan was to be able to support up to 98 U-Boats with 30 at Lorient, 20 each at Brest and Lorient, 15 at Bordeaux, and 13 at La Pallice.
The sheer scale of the sites meant they were easily spotted by Allied aircraft and frequent bombing raids took place. However, few of the raids were effective at breaking through the 7m thick, steel-reinforced roofs. These roofs were also protected by ‘Fangrost’, a layer of reinforced concrete beams set in rows with spaces in between to create blast chambers which would dissipate the energy of bombs, including the famous ‘bunker-busting’ Tallboy.
It’s this sheer strength which ensures the bunkers are still visible today.

Bordeaux U-Boat bunker
The Bordeaux U-Boat base differs from the other four sites in western France in that this is the only one which is actually inland of the coast.
Located around 90km upstream of the mouth of the River Garonne in the famous wine region, the U-Boat site stands in the heart of the city, connected to the large tidal river by a once defended lock gate.
Bordeaux was originally used by the Italian navy following the capture of France by Axis forces in 1940.
By September 1941 work had begun on creating a multi-cell bunker for protecting, servicing and resupplying German U-Boat flotillas. This site was home to the Kriegsmarine’s long-range submarines and the ‘milch cows’ which were re-supply boats that would rendezvous with U-Boats in the Atlantic to deliver supplies and torpedos as far away as the US coast.
The bunker built featured 11 pens with three wet docks and eight dry docks. It measured over 240m wide by 160m long and was constructed with a steel reinforced concrete roof of 5.6m thickness and was able to protect up to 15 submarines at any one time.
Surrounding the main facility, you can find a large torpedo storage bunker and an even bigger fuel depot, thought to have been able to store around four million litres of diesel which would have not only been used for U-Boats, but also to fill the storage tanks of the milch cows.
After Italy capitulated in 1943, German forces seized the few remaining Italian submarines at the site, including ones which were unseaworthy or not capable of fulfilling their requirements. It was one of these Italian boats, UIT-21, which was found inside one of the pens along with U-Boats U-178 and U-188 when the site was captured on August 25, 1944. The three submarines were scuttled a few days later.
One of Bordeaux’s last visitors was U-534 which left the base on August 22 to avoid being captured and made its way to Denmark. On May 5, 1945, it was attacked and sunk off the Danish coast by Allied aircraft but was salvaged on August 20, 1993, and was put on display – albeit in cut-up sections - on the banks of the River Mersey in Birkenhead, England. This large-scale museum exhibit is currently undergoing refurbishment before going on display again to the public within the next few years.
Today the Bordeaux site is in industrial usage but there is also a part which houses a museum, exhibition space, and art gallery which can all be visited.