Few things of the Second World War are more intimidating than the iconic German Bunker. Made out of reinforced concrete with a thickness of up to 3.5 meter, these casemates and pillboxes were incredibly tough to destroy. Built to withstand shells and bombs, they provided shelter to troops and civilians alike. But there were also some even larger super-structures. From giant U-Boat shelters and fearsome Flak-Towers, to the ultimate Führerbunker, the Germans perfected the art of bunker building.