The vault is used to store a large portion of the United States' gold reserves as well as other precious items belonging to or in custody of the federal government.

During World War II the signed original Constitution of the United States, Declaration of Independence, Articles of Confederation, Lincoln's Second Inaugural Address and drafts of Lincoln's Gettysburg Address were stored in its vault for protection, as was a Gutenberg Bible and an exemplified copy of Magna Carta.

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.

[3] In June 1935, the U.S. Treasury announced its intention to quickly build a gold depository on the grounds of Fort Knox, Kentucky.

This intent was in keeping with a policy previously announced to move gold reserves away from coastal cities to areas less vulnerable to foreign military invasion.

An army attacking from the Eastern Seaboard would have to fight through the Appalachian Mountains, which were considered a reasonable impediment to military forces of the time.

Finally, the Army's only completely mechanized cavalry unit was stationed at the adjacent Fort and could readily be deployed to defend the depository.

[4][6] The first wave of gold shipments was made semi-weekly between January 11 and June 17, 1937, and overseen by the United States Post Office Department.

[7][8] The gold was transported from the New York Assay Office and the Philadelphia Mint onto trains using postal trucks and municipal police escorts.

[8] In the armored train cars, postal workers were accompanied by soldiers, secret service agents, and mint guards.

[8] The gold was transferred from trains onto Army trucks under the protection of soldiers armed with armor-piercing bullets and machine guns.

[8] On March 1, 1941,[12] United States Secretary of the Treasury Henry Morgenthau Jr. announced the completion of another shipment totaling 258.74 million troy ounces (8,048 metric tons) of gold from the New York Assay office to the depository.

[13] The building and early operation of the depository occurred at the same time total gold reserves in the United States experienced unprecedented growth.

Factors driving this growth included the gold price revaluation (dollar devaluation) in 1934 spurring a rise in global gold production, political uncertainties in Europe causing a capital flight to the United States, and re-armament programs in Europe which increased U.S. net merchandise exports.

This consisted of 174 million troy ounces (5,421 metric tons) out of foreign mines (mainly from South Africa), 89 million troy ounces (2,755 metric tons) out of foreign central bank reserves (mostly France and the United Kingdom), with the balance from other sources (principally private holdings in India).

[15] By the end of 1940, total Treasury reserves stored at all locations rose to 628.4 million troy ounces (19,546 metric tons).

[19] As the Battle of Britain was fought during the summer and fall of 1940, MacLeish asked the U.S. Geological Survey about locating underground storage for "valuable paintings and books" and "within reasonable distance of Washington.

"[20] In December 1940, he directed his staff to create a detailed catalog of the Library of Congress's most "irreplaceable" assets, and the space required to store them.

[22] On April 30, 1941, he requested of the Treasury Secretary some thousands of cubic feet at Fort Knox for the most notable items in the library.

[24] While the vault was invulnerable to bombing attack, it was not climate controlled, and so the documents were vulnerable to changes in temperature and humidity, as well as insects.

They were given to U.S. military authorities by members of the Royal Hungarian Crown Guard who feared that they would otherwise fall into Soviet hands.

[32] In 1955, the Defense Logistics Agency began storing opium and morphine at the depository and the West Point Mint.

The stockpile grew to 68,269 pounds (30,966 kg), enough to meet the legal painkiller needs of the entire United States for one year if supplies were cut off.

As the Cold War ended, and more nations began exporting concentrated poppy straw, concerns about supply disruptions abated.

The tour was in response to a conspiracy theory, circulated by Peter Beter, that elites had secretly removed the gold in the depository and that the vaults were empty.

[40] The third inspection tour was on August 21, 2017, when Kentucky Senator Mitch McConnell visited with a small group of officials and staff, including Secretary of the Treasury Steven Mnuchin.

[42][36] According to a tweet by Mnuchin, and an internal email by Chief of the U.S. Mint Police Dennis O'Connor, he was the first Treasury Secretary to visit the depository since John Wesley Snyder in 1948.

[60] Given its reputation for securely holding large amounts of gold, breaking into the depository has been featured in many popular books, movies, games, and television shows.

[58] In the 1951 comedy Comin' Round the Mountain, Abbott and Costello follow a treasure map and unwittingly dig into the vault at Fort Knox, where they are immediately arrested.

[58] In the 1952 animated cartoon 14 Carrot Rabbit, Bugs Bunny tricks Yosemite Sam into digging into the vault, where he too is immediately arrested.