The U.S. Department of Defense is spending nearly a billion dollars to upgrade the old U.S. Air Force base inside Cheyenne Mountain. Built in the early 1960s, this complex is literally inside a mountain and during the Cold War was the NORAD (North American Air Defense Command) headquarters, in large part because the base was considered very difficult to destroy, even with nuclear weapons. During the two decades after the Cold War ended (in 1991) the air defense system shrank and NORAD moved out to an underground facility at a nearby airbase. The new Space Command became the principal tenant in Cheyenne Mountain. But because of the fear of enemies using EMP (Electromagnetic Pulse) weapons to destroy electronics many military organizations want to get some of their operations inside Cheyenne Mountain because this place was designed to be EMP proof and is still the most EMP proof military facility the United States has.
Since the 1950s it was known that the EMP put out by nuclear weapons could damage or destroy solid state (transistors and microelectronics) devices. Since the 1990s devices using high-powered microwave (HPM) devices have been developed to create focused EMP on demand without all the nuclear blast and radioactivity. The most commonly mentioned devise to generate HPM is the AESA (Active Electronically Scanned Array) radars that are becoming standard equipment in modern warplanes. AESA is more reliable and, increasingly, no more expensive than the older mechanical (a small dish that moves around inside a dome) radar. AESA is also easier and cheaper to maintain, which makes a more expensive AESA cheaper, over its lifetime, than a cheaper (to buy) mechanically scanned radar. More and more nations (including China and Russia) are manufacturing AESA radars and equipping their ships and aircraft with this stuff. All these nations are also manufacturing or developing EMP “bombs” that could be used to sabotage military bases or civilian facilities. For a long time EMP was believed to be an unlikely threat because you needed a nuclear war to create EMP. Naturally the blast and radiation damage from the nukes was seen as more of a threat than EMP. But now that has changed.
China, Russia, North Korea and Iran already have lots of underground facilities for protection from all sorts of unpleasantness, including EMP. Thus Cheyenne Mountain is again becoming the major headquarters for the defense of North America (NORAD and Northcom).