This is a bit special base as it's an inland port, 60 miles from the coast.
From the autumn of 1940 Bordeaux had been the base for the Italian Betasom submarine flotilla. The first 3 Italian submarines entered the base in September, 1940. On October 9, 1940 the first Italian submarine Malaspina left the base for a Atlantic patrol and few days later operated against the convoy OB-229.
Roughly 25 Italian submarines were attached to the flotilla at one time or another, but attempts to stage an effective cooperation with the German wolf pack attacks failed for the most part and sinkings for the Italian boats were much lower than of their German allies.
Admiral Dönitz decided during the summer of 1941 to also a build a protective bunker in Bordeaux. Construction began in September 1941.
The bunker was 245m wide, 162m long and 19 m high. Above the pens the roof was 5.6m thick and 3.6m thick above the rear servicing area. The first U-boat to use the bunker was U-178 on January 17, 1943.
The base became the home of most of the supply boats, the milk cows of type XIV, and the very long range boats of type IXD2 belonging to the 12th Flotilla. Also the mine-layers of the XB and the torpedo transports of type VIIF were stationed there.
The 12th Flotilla took over 5 Italian submarines, converted them for transport duties, in September, 1943 after the Italian capitulation.
The last U-boats to leave the base were U-534 and U-857 which left to Norway/ Germany 3 days before the allies occupied the base on August 25, 1944, reaching Flensburg in the end of October 1944. Two U-boats could not retreat from the base before the occupation, the IXD2 U-178 and the IXC/40 U-188 which were not seaworthy and were scuttled on Aug 20.
One U-boat was found in the pens when they were entered, the former Italian boats, UIT-21 which had been scuttled there, never put into use by the Germans due to its poor shape.
The bunker is today in private use. A permission to visit is possible without problems.