All U-Boats communicated back to the mainland and most of the Enigma-encrypted messages went to a huge communications site at Basse-Lande, around 10km to the southwest of Nantes.
This site was a former French Navy radio station – built in 1919 and modernised in 1937 – which was captured by the advancing German forces. It consisted of 10 antennas of 180m height based around a central communications HQ.
Capable of sending and receiving messages from as far as the coast of the US, this site played a crucial role during the Battle of the Atlantic where it would relay information on the positions of Allied convoys in the Atlantic Ocean, allowing U-Boat wolfpacks to target and destroy ships containing vital supplies bound for the UK.
However, by June 1941 the Allies were able to intercept the site’s messages via by a number of special receivers located on the south coast of England and these would be passed on to Bletchley Park for decoding and transferred to the British Naval HQ at Western Approaches in Liverpool for action.
Today the Basse-Lande site stands on private land and cannot be accessed by the public but you can see the former HQ building – now derelict - and water supply tower from the entrance road around the large site.