In 1958, at the behest of the Eisenhower administration, the Greenbrier facility was designed to shelter the Congress in case of a nuclear fall-out. It was maintained as a fully operational self contained unit for 30 years, until Washington Post published an article revealing all, going against congressional requests to desist. The next day began the deactivation of the facility, and it was returned to the Greenbrier in 1995. Go read the article, its pretty cool.
The bunker was built under the West Virginia wing of the hotel, the wing built first to serve as camouflage for the bunker. The story was that an exhibition hall and two auditoriums were being built under the West Virginia wing. This was true, but these were actually parts of the bunker, that would be used for stated purposes. In case of nuclear fall out, they would be closed off into the bunker and become congressional offices and chambers for the House and Senate.
There were four entrances to the bunker, all shielded by thick steel doors built by Mosler safe company and filled with reinforced concrete.
The senators were to be evacuated to the Greenbrier by road, train or plane, then decontaminated and housed in dorms. There were slightly better rooms with twin beds for the leaders of the senate.
From the outside, its hard to tell where the bunker is, with the elaborate layout of the place and all the landscaping!