German submarine activity commenced in 1940 at the main Arsenal, however, the site was vulnerable to attack. At the end of 1940, the Todt organisation established a team at Lorient to find a suitable site to build a base to careen, repair and re-supply the submarine fleet. After undertaking several surveys as well as dredging the roads, the choice was made to use the Keroman peninsula. So the Germans requisitioned the 20 hectare (50 acre) site.

Work was carried out from 1941 through to 1944 using a workforce of 15,000 men from France, Holland, Belgium, Spain, Portugal, Italy and Morocco and 2,000 trucks. They started, in 1941, with Block K1 (Keroman 1). The hard rock upon which the block was built prevented rapid construction of wet dock facilities so they decided to use a slip-way/dry-dock system to haul the submarines out of the water. The design of K1 allowed for submarines to be berthed in the heavily protected slip-way, then, as the water was pumped out, lowered onto a transportation cradle which was then winched up the inclined plane of the slip-way until the cradle and submarine were at the same level as the floors of the pens. The cradle and its submarine were then moved to a vacant pen. The first submarine to use the system was U123 on 25th August 1941, even though the building of K1 was not completed until September 1941. K1 has 5 pens each fitted with a pair of massive blast-proof doors.

Work commenced on K2 in May 1941, in parallel with the work on K1, and continued until December 1941. K2 has 7 submarine pens. Both blocks were supplies with electricity, compressed air, diesel oil, sea-water and fresh water through a system of underground concrete tunnels. An extremely well protected, diesel powered electrical generating station ensured a constant supply of electricity.

In October 1941, work commenced on the construction of Block K3 to the south-east of the K1 and K2 complex. This concrete shelter is of a different design to the other two having 7 pens all with deep-water access and able to be used as both wet and dry docks.

Digging for K3 was made easier because the ground was silt rather than rock, even so, construction was a more delicate process which, hindered by the numerous allied bombardments was not completed until January 1943. The pens were sealed with enormous floating armoured doors. The roof is 3m or reinforced concrete on top of which is an additional set of 3m high chambers running the length of the block and surmounted with 1m2 reinforced concrete strips ? falling bombs would impact on the strips and detonate, the blast being dissipated along the chambers ? the main roof would remain undamaged.

In 1944, a fortified station and barracks were under construction but these remained incomplete by the time the base surrendered on 10th May 1945 ? 2 days AFTER the war ended!

Lorient was the largest of the 5 German Atlantic Coast bases in France, of 1149 careenings which took place in France, 492 were undertaken in Lorient.