The Bunker de la Rochelle is a small, peculiar museum hidden underneath what is now a private courtyard a few streets back from the harbour. The bunker was built by German engineers in 1941 as a refuge and operations room for the staff of the Atlantic U-boat command stationed at La Pallice. It has been preserved as it was when the Germans abandoned it in 1945, complete with bunks, gas-tight doors, ventilation systems, and original signage.
The museum is small but vivid — visits are guided in French (English audio guides available) and take about an hour. Combine with a visit to the larger U-boat pens at La Pallice for the full picture. The contrast between the polite town centre above and the concrete claustrophobia below is the point.