Fort Knox, Kentucky

The U.S. effort to build new military installations after WWI led to the start of Ft. Knox in 1918. Over years, the security and fortification has continued to increase, especially as the fort became home to the United State Bullion Depository, home of the largest portion of the country's official gold reserves. That security starts with an open field and includes layers of granite, concrete, and steel barriers, and that's before you even get to a safe that was built to withstand atomic bombs. Add in the presence of the U.S. Army, digital and visual surveillance, and the potential for every high-tech form of defense imaginable (there are fun rumors of flooding tunnels) and you get America's most secure safe. Of course, nobody really knows how much gold—or valuables of all sorts—is located within the bowels of Fort Knox.