Deep underground in Butler County, the Iron Mountain National Data Center functions like its own little city. With more than 3,000 badged employees, the former mine has its own fire department, restaurant, transit systems, water treatment center, and medical center.

But what may be most surprising about this unique facility, located outside of Pittsburgh in Boyers, is the treasures that are stored inside.

Formerly a limestone mine owned by United States Steel Corp. (NYSE: X) in the early 1900s, today the mine houses everything from original prints of classic motion pictures, to data storage for more than 2,300 business and government agencies across the U.S.

Located 220 feet underground, Nick Salimbene, director of business development for Iron Mountain, said what makes the facility so unique is that it is virtually impervious to natural disasters. Salimbene said being housed underground also provides a stable environment that naturally maintains a temperature of 52 degrees.

Of the 145 acres that are developed, Salimbene said about 70 percent is used for physical storage. But demand is shifting toward more data storage. He said the demand for data storage has been increasing between 20 percent and 30 percent annually in recent years. Today, he said about 80 percent of what is coming into the center today is data.

For the privacy of its customers, Iron Mountain does not disclose who uses the Boyers facility by name. But it includes many big names. One tenant Salimbene could discuss is Corbis Corp., which houses its collection of over 20 million photographs .

"There's a little bit of everything here," Salimbene said. "But the most important thing for us, is that our customers feel secure having their items located here."

Obviously, with such valuable objects in its facility, security is very tight at Iron Mountain, with armed guards keeping watch 24 hours a day, seven days a week.

"We have companies trusting us with their most valuable assets," Salimbene said. "That's not something we take lightly."