I'm reading Sonia Purnel's book 'A Woman of No Importance' which deals with the life and times of Virginia Hall, an S.O.E. operative in France during WW2. In passing, the author mentions Hall's involvement with a U-boat bunker in Marseille. I was unaware such a bunker as are many web sources such as Wiki. Some research revealed its presence.
'Marta' was started in early 1943 and never completed. It was to harbour 20 unterseebooten. It was bombed on 27 May, 1944 by the USAAF. Offshore the Germans fortified the Iles du Frioul archipeligo with several gun batteries in order to protect the port. The island was also heavily bombed and shelled by the U.S.S. Nevada.
Currently 'Marta' is becoming an undersea cable data centre!