Fort Knox was an establishment containing Earth's largest gold depository before the New World Economy took form. It was located in the state of Kentucky of the United States of America. At its peak, it held upwards of 50 metric tons of bullion. It became a museum after the New World Economy formed. It was considered virtually impenetrable despite an attempt by Ferengi to break in during the late 24th century.
In 2375, while planning a heist of a Borg transwarp coil, Captain Janeway of the USS Voyager used Fort Knox as a metaphor to explain the scale of the prize they sought. Tuvok did not understand what Janeway meant, requiring Tom Paris to explain. (VOY: "Dark Frontier")
In actuality, Fort Knox is not the largest repository of gold bullion in Earth's history but rather the Federal Reserve Bank of New York.