COLO SPRINGS, Co. - In response to fears associated with the Cold War, Cheyenne Mountain Air Force Station was built in the early 1960's to withstand a nuclear explosion.
An emerging high tech threat, however, has created new value in this manmade cave designed more than half a century ago.
The original tenant of the mountain, NORAD, still maintains an alternate command center in the underground complex, but in 2006, the agency officially moved its headquarters to Peterson Air Force Base.
That move generated talk about shutting down Cheyenne Mountain entirely.
"There was a lot of vacant space in the mountain, and there were some conversations in those times, the Rumsfeldt era, about whether or not to shutter Cheyenne Mountain," explained Colonel Gary Cornn Jr., the commander of 721st Mission Support Group in charge of operating and security the installation.
However, the increasing threat from countries like North Korea and Iran, who are determined to develop nuclear weapons and the missiles that carry them, has reversed that talk.
The new fears are not associated with the classic mushroom-cloud explosion associated with nuclear weapons, but rather an EMP blast.
The detonation of a nuclear missile at the right spot above the earth could create an invisible but powerful electromagnetic pulse that could destroy not only every unprotected electronic device in the country, but the power grid itself, taking decades to recover from.
Col. Cornn says the 2,000 feet of granite, along with more subtle features of the mountain complex, acts as a shield against an EMP.
On a normal day, Cheyenne Mountain Air Force Station draws electricity from Colorado Springs Utilities, but the military is able to cut the cord almost immediately and switch over to internal generator power in a matter of moments if needed.
"There's a capability that we have to disconnect from the grid, and that allows us to save up from an EMP threat," says Cornn.
As a result, the head of NORAD and NORTHCOM confirmed in 2015 for the first time that a number of groups have moved equipment and staff back into Cheyenne Mountain, specifically for its EMP protection.
Admiral William Gortney, now retired, said, "It wasn't really designed to be that way, but the way it was constructed makes it that way, so there's a lot of movement to put capability into Cheyenne Mountain and to be able to communicate in there."
Admiral Gortney wouldn't confirm which groups have sought space in the mountain, but the fact that he can't suggests they are among the most critical to the nation's defense.
The reality of working at a facility that continues to play such a critical role in the defense of the nation is a constant source of pride for the men and woman stationed at the mountain.
"It's more of an honor for me to be here. I mean this is America's Fortress," said Airman 1st Class Scott Smith, who guards the blast doors at a member of the 721st Security Force Squadron.
"I'm defending the United States and North America. It's quite a privilege to be here," he added.
Staff Sergeant Courtney Maikowski of the 721st Communications Squadron said, "I wouldn't get this opportunity in the civilian side, so this is a once in a lifetime thing for me, and it makes me want to continue doing this job."
Chief Master Sergeant Scott Robbins, who recently took over as Command Chief of the 721st Mission Support Group, said it only took him a few days in the mountain to recognize their dedication.
"They are proud of what they do. They are proud of the mission," he said.
As the command staff puts it, Cheyenne Mountain AFS was "built for the worst day in American history", and today it still serves that purpose, even though it's a bit less obvious what exactly that "worst day" will bring.
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