Iron Mountain Boyers, Pennsylvania

This former limestone mine is one of the more notorious underground sites in the nation, covering more than a thousand acres underground, with up to two million square feet of developed space. In addition to storing films and documents for many national companies, it has been the location for one of the largest image archives in the world, generally known as Corbis, once owned by Bill Gates, but now merged into the collections of its former rival, Getty Images. The facility was originally owned by US Steel, which from 1902 to 1952 excavated limestone for use in steel mills around Pittsburgh. The largest tenant in the complex has been the US Government, which started storing records here in 1960, and has at times employed more than a thousand people inside the mines, working for the Office of Personnel Management, the US Patent and Trademark Office, US Investigations Services, and others. In 1998 the mine's owner, National Underground, based in the Pacific Northwest, was purchased by Iron Mountain, now the largest records storage company in the nation, with over 100 locations, only a few of which are underground. Iron Mountain has recently expanded its data center, known as WPA-1, inside the mine.