Along France's Atlantic coast, Germany built vast concrete bunkers for its U-boats during the Second World War. Bombers could not break them, and today they are museums, art galleries, naval stores and silent ruins.
Built on a river island in the 1830s and 1840s, the fortress at Brest hosted the treaty that took Russia out of the First World War, held Polish opposition leaders in the 1930s, and was attacked in the first hour of Operation Barbarossa. Its ruins now stand inside a vast Soviet-era memorial complex.
