NORAD is the North American Aerospace Defence Command, controlled from within Cheyenne Mountain near Boulder, Colorado. This facility was used as America's first defence to an impending attack, with high-tech equipment. How long will it last?
4 years after people: Due to a lack of maintenance the water resevoir in the Mountain is leaking. The interior "buildings" are constructed on springs which isolate them from the Mountain movement, but the springs can corrode.
4,000 years after people: NORAD remains recognizable, the flat-panel computers and desks will have accumulated a lot of dust though.