9. Fort Knox, United States
The United States Bullion Depository, known as Fort Knox, is located in Kentucky and houses a large portion of the American gold reserve. The vault housing the gold, which is said to total 3% of the total supply ever refined in human history, is lined with granite walls and protected by a 22 ton door that can withstand bomb blasts. A full list of the fort’s contents cannot be found, as the last known audit of the facility was done in 1930. It has been home to the U.S. Declaration of Independence as well as the Constitution and has held copies of the Magna Carta. Today, it protects valuable items entrusted to the federal government and is guarded around the clock by the U.S. Mint Police.