The former military facilities can be visited
La Pallice Submarine base

In La Rochelle – La Pallice in the Nouvelle-Aquitaine region of France, the U-Boat Bunker La Pallice stands as a structure of military architecture from World War II. It features massive construction and intact preservation.
History+
The German U-boat fleet used the port of La Pallice as a strategic base from 1940. Construction of the bunker complex began in spring 1941, and within six months, it included a covered lock and seven U-boat berths. The bunker is 192 meters long and 159 meters wide, featuring a 7.3-meter-thick double reinforced concrete roof.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Baselapallice08.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 46.1585, -1.2093