Visitors must book two months ahead to attend public briefings that consist primarily of a multimedia presentation. Security is tight; entrance is not permitted to the Cheyenne Mountain Operations Center itself. In the early 1960s, 693,000 tons of solid granite were carved from within the peak to house the North American Aerospace Defense Command (NORAD) and US Space Command, in a facility insulated against earthquakes and nuclear explosions. Today, 1,450 employees work underground in 15 independent buildings, each with its own tunnel.