This is known at the Library of Congress Packard Campus and is home to National Audio-Visual Conservation Center. The site was once known as Mount Pony - Federal Bunker.
Good pictures here.
The Federal Reserve Board operated a 140,000 square foot radiation hardened facility in Culpeper, Virginia. Dedicated on 10 December 1969, the 400 foot long bunker is built of steel-reinforced concrete a foot thick. Lead-lined shutters can be dropped to cover the windows of the semi-recessed facility, which is covered by 2 to 4 feet of dirt and surrounded by barbed-wire fences and guard posts. The seven computers at the facility, operated by the Federal Reserve Bank of Richmond, are the central node for all American electronic funds transfer activities. Until July 1992 the bunker also served as a Continuity of Government facility. With a peacetime staff of 100, the facility was designed to support an emergency staff of 540 for 30 days. But only 200 beds were provided in the men's and women's dormintories, which would be shared on a "hot-bunk" basis by the staff, working around the clock. Until 1988 the facility stored a $1 billion stock of currency to be used to reactivate the American economy following a nuclear attack.
http://en.wikipedia.org/wiki/Library_of_Congress#Packard_Campus_for_Audio-Visual_Conservation
http://www.boingboing.net/2010/06/09/gallery-digitizing-t.html