In 1944, the Nazis built a monstrous underground complex in France, then filled it with over 12,000 tons of steel meant for a weapon the world wasn’t ready to face. Allied bombers tried to erase it from existence, but decades later, parts of it still remain untouched. What’s more disturbing? Some of the files about it are still sealed. This isn't just a forgotten relic. It’s a military mystery buried beneath your feet.