La Base Sous-Marine Bordeaux

By far the city's eeriest and most menacing sight, this mammoth hulk of a submarine base (1941–43) was one of five built on the Atlantic Coast by the Germans during the WWII. Designed as a bunker to protect German U-boats from aerial attack, it pens base proved impossible to destroy – by British forces during WWII and subsequently by the city, who now use the eyesore reinforced-concrete structure as a seriously cool, underground cultural centre, art gallery and music concert venue. The submarine base is only open during exhibitions and events. Check its Facebook page for details.