When you think of Fort Knox, you can’t help but think about gold. That’s with good reason. The U.S. Bullion Depository at Fort Knox, Kentucky is the primary location of the United States’ gold reserves and other precious items. It is not, however, the largest gold depository.

Unbeknownst to most of the millions of people who walk its streets, the biggest pot of gold in the world lies below the sidewalks of Manhattan, New York. The Federal Reserve Bank Depository in Manhattan holds a whopping 212 million ounces—about 6,700 tons of gold. In contrast to Fort Knox’s holdings of more than 147 million ounces of gold, it is easily the largest gold bullion reserve in the world. The estimated value of the holdings is about $401,528,000,000.

The Federal Reserve Bank Depository’s stash of precious metal is located 25 meters (80 ft) beneath the streets of Manhattan. The level of security is beyond advanced. The only access is closed off by a 2.7-meter (nine-foot) high, 90-ton steel cylinder set into a 140-ton frame. The vault itself is completely surrounded by steel-reinforced concrete walls. Every inch of the facility is under nonstop surveillance, including motion detectors, closed-circuit cameras, and a lot of cross-looking, heavily-armed guards.

Although the gold is held there, none of it belongs to the Reserve. It is held there for other governments and banks. At its peak, the Reserve held over 12,000 tons of gold. According to the Federal Reserve Bank of New York website:

Gold bars are transported by elevator from street level to the vault’s basement location. Once inside the vault the bars become the responsibility of a control group consisting of three representatives: two members of the New York Fed gold vault staff and one member from the New York Fed internal audit staff. These three individuals must be present whenever gold is moved or a compartment is opened in the vault—even to change a light bulb. This helps ensure proper safekeeping and maximum security for the gold.

All bars brought into the vault for deposit are carefully weighed, and the refiner and fineness (purity) markings on the bars are inspected to ensure they agree with the depositor instructions and recorded in the New York Fed’s records. This step is vital because the New York Fed returns the exact bars deposited by the account holder upon withdrawal—gold deposits are not considered fungible.

Following the verification process, the gold is moved to one of the vault’s 122 compartments, where each compartment contains gold held by a single account holder (meaning that gold is not commingled between account holders). In rare cases, small deposits are placed on separately numbered spaces on shelves in a “library” compartment shared by several account holders. Each compartment is secured by a padlock, two combination locks, and an auditor’s seal. Compartments are numbered rather than named to maintain confidentiality of the account holders.

The New York Fed charges account holders a handling fee for gold transactions, including when gold enters or leaves the vault or ownership transfers (moves between compartments), but otherwise does not charge fees for gold storage.

The vault is open to the public and there are regular tours. The tours are free and you can register for one here.