Culpeper Switch was built in 1968 as a continuity of operations site for the United States Federal Reserve. In addition to acting as the center of the Fedwire electronic funds transfer system, Culpeper also had $4 billion in cash and special facilities to allow it to function as a backup Federal Reserve headquarters. After the Cold War the facility was shut down and was purchased by the Library of Congress to become the National Audio-Visual Conservation Center. There's more information on Culpeper at this blog, and an informational brochure about it here. Photo of the site in 1990, from Wikipedia. Some photos from the brochure: