The United States Bullion Depository is located at the fort and became active in the 1930s, housing a large number of the federal gold reserves. At some point before the Great War, all the gold from Fort Knox was secretly moved to Vault 79 for storage in the case of nuclear war, and Fort Knox was left retired as a depository, completely empty of gold bullion.
Behind the scenes
Fort Knox is a real-world place, a United States Army post in Kentucky. The fort is adjacent to the United States Bullion Depository, which is used to house a large portion of the United States' official gold reserves and is often mistakenly referred to as Fort Knox in popular culture.