The United States Bullion Depository — known as Fort Knox — is considered the most heavily guarded place on the planet. Between its fenced perimeter and granite-lined concrete structure lie rings of razor wire. The grounds are monitored by high-resolution night vision video cameras and microphones, and the subterranean vault is made of steel plates encased in concrete. Its torch-and-drill resistant door is 21 inches thick and weighs 18 metric tons. The vault door is set on a 100-hour time lock, and can only be opened by members of the depository staff, who must dial separate combinations. Visitors are not allowed inside. The facility is so secure, in fact, that the phase “Fort Knox” has become synonymous with safety and security.