During WWII Hitler knew that an Allied invasion was inevitable. His plan included hundreds (?) of fortified concrete bunkers built all along the Nazi-controlled coastline with the English Channel and North Sea. These Bunkers were dotted along several regions from the French Coast all through Scandinavia.
In addition to bunkers he also had projects that included the largest land-based ground weapon... ever. That massive gun (still visible on google maps, but now I can't find the link) actually started out as plans for an absolutely massive tank. The turret portion was built but the vehicle portion was deemed unfeasible, that's where the plans to install it in a fixed position arose.
Due to the sheer scale and features like 9-foot thick walls in some places, most of these structures, while covered in graffiti, vegetation or converted for other uses by local residents are still recognizable. Click the source links for more.
Sources -> http://www.wired.com/rawfile/2013/02/world-war-ii-bunkers-jonathan-andrew/ &