The U-boat pens at La Pallice — Base sous-marine de La Rochelle — are one of the largest Nazi concrete structures still standing, an industrial monster of 195,000 cubic metres of reinforced concrete on the western edge of the city. From here, between 1941 and 1944, U-boats sailed out into the Atlantic on the patrols that nearly cut Britain off from the Americas.
You cannot go inside — the structure is still a working military zone — but a viewing platform and information panels on the outer perimeter let you take in the scale. The pens were the setting for the climactic chase in Wolfgang Petersen's Das Boot. The road to La Pallice runs west from the centre; you pass it on the way to Île de Ré. Combine with the Bunker de la Rochelle museum in the centre for a fuller wartime picture.