The facility is in Boyers, Pa (not far from Pittsburgh). It's a former limestone mine in a very hilly (but not necessarily mountainous) area. It is owned by a private company and the US government, among others, stores records there.
A TV news piece on it:
http://www.youtube.com/watch?v=2aou6c2MOmg
I found this about it on the net:
A long Washington Post article about Boyers, published in the mid-1990s, began this way: “Ten thousand years from now, space aliens are doing to discover the catacombs of Boyers and think they have hit an archeological jackpot: Millions of government documents stored in acres of filing cabinets 250 feet below the Earth’s surface, safe from flood, fire, famine, Iraqi nerve gas, nuclear holocaust and roaches. . . . OPM [Office of Personnel Management] started to move records to Boyers in 1960, a few years after National Underground Storage, Inc., a Pittsburgh company, bought the mine from U.S. Steel and converted it to a secure facility so individuals, companies and the U.S. government would have a safe place to keep things. . . . The only thing above ground is a parking lot and a tunnel mouth wide enough to accommodate a railroad car.â€
As far as earthquakes, strangely earthquake "waves" move along the earth's surface down to a depth of about 10-15 feet. In the 1950s or 60s, a pair of scientists volunteered to go into Carlsbad Caverns, 700 feet below the surface, during a nearby underground nuclear test. The felt nothing and came back up and asked why the test had been canceled.