Saint-Nazaire U-Boat Pens

Following the surrender of France to German forces later in June 1940, the port immediately became a base of operations for the Kriegsmarine (German Navy) and was as such the target of Allied operations. A heavily fortified U-boat submarine base was built by Organisation Todt shortly after occupation, with its 9 m (30-ft) thick concrete ceiling was capable of withstanding almost any bomb in use at the time.

http://en.wikipedia.org/wiki/St_Nazaire,_France#U-Boat_pens