On 26 June 1940, France falls after a brief battle with German forces. Two days later, Grossadmiral Karl Donitz authorises the construction of a massive submarine base in Lorient, Bretagne. The Battle of the Atlantic is about to begin.

The Story

With the fall of France, the German high command became convinced that blockading the United Kingdom out of supplies would force the only country that stood between them and total victory in Europe into capitulation. With the British isles already cut-off from the rest of the continent, controlling the Atlantic shipping lanes that kept Britain supplied from overseas would deliver the final stranglehold.  But the Germans feared a full naval confrontation with Britain. The so called Plan Z, supposed to produce a powerful surface fleet capable of challenging the Royal Navy, had never fully materialised. And so Germany’s meagre surface fleet spent the course of the war overstretched,  repairing in port or, as it were, in the bottom of the sea.

Type VIIB U-47 and the Battlecruiser Scharnhorst in late 1939

They did however soon embrace unrestricted submarine warfare as a means of controlling the crucial Atlantic shipping lanes. Because of the lack of an effective surface fleet, attacking merchant shipping while remaining undetected soon became the cornerstone of Germany’s strategy in the Atlantic. To realise this strategy, they commissioned a terrifying 1250 U-booten in the Kriegsmarine during the course of the war. And to use them with maximum effect,  they built 5 submarine bases in France’s Atlantic coast : In Brest, St.Nazaire, La Rochelle, Bordeaux, and the largest of them all, the Keroman Base in Lorient : a beastly mountain of concrete capable of sheltering 30 submarines. The dramatic naval confrontation that ensued was to become the longest ongoing operation of the war. It became known as the Battle of the Atlantic.

The Keroman Uboot Base
View from the sea of K3 and the K1 entrance (left) of the Keroman Uboot Base in 2011 by © XIIIfromTOKYO

The Organization Todt, Nazi Germany’s main engineering group, undertook the construction task. After surveying the lay of the land at the Keroman Peninsula near Port Lorient, they decided to build the base there. The first bunker built on Keroman Peninsula was named K1, and 15.000 workers from France and elsewhere in Europe were employed on the site for 11 months, pouring tremendous amounts of reinforced concrete to build it. Work would soon begin on its extension, the K2 bunker just across the first.

Building the second section at La Pallice Base, La Rochelle, 1942

However, the bedrock of the Keroman peninsula prevented them from digging deep to create wet dock pens for all the submarines. Instead they engineered an ingenious system : a single wet dock entrance and slipway, combined with a mechanical cradle system that took the submarine out of the wet dock and placed it on a trackway. The submarine could then be transported and assigned to an empty dry pen in either the the K1 or K2 protected bunkers. Imagine this as a sort of jukebox inspired mechanism, where U-boots are the records, safely stored in pens until one is selected and moved in and out of the play area – the wet dock. The whole operation, from cradling the boat in the wet dock to safely storing it into its dry pen could be completed within only 15 minutes !! A further extension was built later in the war, the K3. This new bunker contained another 7 pens, this time with either wet or dry capability. Construction began for another major extension to the base, the K4, but it was never completed due to lack of supplies and materials.

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Some of the greatest U-boot commanders of the war would operate out of Lorient : Otto Kretschmer, the so called “Silent Otto”, who sank a staggering 47 ships or 274.000 tons of shipping within 14 months before his U-99 was attacked and disabled by British destroyers. He surfaced, scuttled and was captured in March 1941. The daring Gunther Prien, who covertly entered the Royal Navy’s Scapa Flow anchorage in 1939 to sink the Battleship HMS Royal Oak, one of Kriegsmarine‘s astounding early successes. Prien sank another 30 ships, many of these patrolling out of Lorient, for a total of 200.000 tons – before he went missing in the Atlantic in March 1941 along with all hands of U-47. Or Niko Clausen, a long range U boat specialist, who sank 24 ships or 74.000 tons before his U-129 was attacked and sunk by US destroyers somewhere in the Indian Ocean. U-boot crews lived fast and dangerously, and more often than not, they ended up in Davy Jones’ locker..  784 U-boots, over 60% of all submarines the Kriegsmarine built were lost in action. But not before inflicting staggering losses to Allied shipping : German U-boots sank about 3.000 ships, totalling over 14 million tons in all theatres during the course of WW2. That’s about 70% of all Allied shipping losses in the entire war !!

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Stung by these early German successes in the Atlantic,  the Allies soon began launching aerial attacks on the submarine bases in France. But the base at Keroman peninsula was so heavily fortified and protected, that no amount of direct Allied bombing was effective to put it out of action. In the end, the Allies decided to carpet bomb Lorient itself, to cut off the base from supply lines and starve it out of resources. The city was flattened by devastating air raids that destroyed 90% of all infrastructure between January and February 1943. Although this strategy effectively put the base out of operation, its garrison held until the very end of the war, and was one of the last ones to surrender on the 10th of May 1945, already surrounded by US and Free French forces for almost a year.

Allied air raid over Lorient, 1942 : not even a scratch

Today, after a long stint of post-war service with the French Navy,  La Base in Lorient is a national monument. Sensing the futility of trying to dismantle it, local authorities have converted part of it into industrial, museum and exhibition spaces, but also preserved the entire K3 in its original state –  and so it remains an extremely intriguing space to visit.

The Keroman Base K1 wet dock entrance, and beyond it the K3 in the distance

The Visit

I take the motorway exit into Lorient, gripped with anticipation. Since this morning, as for several days now, I’ve been replaying Das Boot in my head : the best submarine war film ever made, shot in the St.Nazaire base about 150km south. The Lorient base, however, is truly so huge that I can see it from the motorway as I approach it. Plenty of parking space near this mountain of concrete too.

Keroman K2
The 18.5m side wall of K2 . Ventilation grilles and a staff entrance

The most obvious place to start the visit is walking at the edge of the pier to take a close look at the K1’s wet dock and ramp. There’s no problem accessing it, as it appears to be used as some sort of pier for small motor boats, and therefore quite accessible.

The K1 is easy to reach and visit

This is where U-boots would sail into, floating atop the submerged cradle, then lowered onto it by pumping the water out, and pulled up the ramp and onto the track way to the safety of the dry pens of K1 and K2.

K1 wet dock and ramp

Next to the K1’s seaward entrance, there’s an ominous looking concrete strong point. Looks like it used to be the business end of an artillery emplacement, but now it’s some sort of disused doorway.

K1 entrance emplacement

The careful observer will still be able to notice some wartime signs of the German presence, such as this stencilled signage by the entrance to K1’s wet dock.                    

Durchgang Verboten (Passage Forbidden)

The humongous steel blast doors that protected the pens are mostly still there. Here’s one of them on K2 :

Steel blast door, K2

The most interesting part of the visit is the K3. To access it you need to walk along the  old track way between K1 and K2. Here’s the one existing side wall of the incomplete K4 – it never materialised.

The side wall of K4, which was never completed

All pens appear partially wet inside the K3. Here is one below. You can see the U-boot access steps on the left and the railings, as well as the wet dock next to it. There’s a lock ahead, keeping the sea waters back, and I suppose a steel blast door externally.


Do you want to know what happens next? Watch this video of me walking through the K3 !

That’s right, this was the original crane from 1941. There was one dedicated to each pen,  used to winch cargo, supplies and torpedoes onto the docked U-boot. Going up the steps, I found the sleeping quarters that seamen could use while at port, along with storage spaces. Going further up, a reinforced steel door leading to the roof of of K3. Then, there’s an amazing sight :

Bomb proof ceiling

I am standing on a section of the internal roof of K3. There are several sections like this, running along the length of the bunker. Above me, there’s another 2 roofs about 1m apart. This is the blast proofing of the bunker : when a bomb falls on the external roof, it begins detonation there, and the subsequent hollow spaces it has to go through means that the blast of the explosion mostly dissipates before the bomb even reaches the internal roof. There’s record of a 5-ton Tallboy Blockbuster bomb hitting the K3 directly in August 1944. It failed to penetrate the bunker, although this was the heaviest airborne ordnance in the Allied armory at the time !!

Flak towers on top of K3

The top side of K3 is very impressive. You can see a number of huge flak emplacements that further protected the base from air attack, as well as the blast proof chambers running along the roof. I spent quite some time on the roof, sneaking around and checking the harbour traffic from above. On the way back, I divert a bit to check out the wreck of SMS Regensburg through one of the K3 pen gates. This defunct wreck of a light cruiser has an intriguing story : It once belonged to the Kaiser’s Navy,  and participated in the monumental Battle of Jutland in 1916 where it acquitted itself honourably, surviving unscathed. It was surrendered to the Allies after the end of World War I,  and subsequently commissioned into service by the French Navy, and renamed Strassbourg. She then joined a number of active combat missions in France’s colonial interventions in North Africa throughout the 20s. The Strassbourg was then involved in an Arctic adventure in 1928. It concluded the search for the vanished explorer Roald Amudsen, by discovering one of his missing seaplane’s floats in the Barents Sea !!  After 1936, it became to old for active service.  It was subesequently decommissioned and then used as a barracks ship at Lorient. This is where the Fall of France found it in 1940, and after a short service as a floating barrage balloon and anti-submarine net platform for the Germans, it was scuttled in front of K3 in 1944 as a barrier against torpedo attacks against the K3 pens’ blast doors. This is her, right there beyond the entrance to K3.


What an amazing place. What an amazing story. Whether you are a U-Boot enthusiast or not, the Keroman U-boot Base is an astonishing place, and combines most of everything you will ever need to see when it comes to the history of submarine warfare during WW2.


Sources :

  • www.uboat-bases.com – a great resource for some of the technical aspects of U-Boot bases in France
  • Uboat.net – an expansive community resource on everything Uboot. Some Kapitän and other relevant data were sourced there
  • The Bundesarchiv  – always a great source for original photo material

  Unless where stated otherwise, all photos and videos are © explorabilia





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