Perimeter Acquisition Radar, Safeguard ABM System (1975). Rising 120 feet above the plain in Concrete, North Dakota, and occupying a full acre at its base, this massive electronic eye, fixed on the northern horizon, was designed to watch for potential signs of an incoming Soviet missile. Originally part of the Safeguard antiballistic missile program, it could detect an enemy ICBM 1,800 miles away as it passed over the North Pole, giving the United States a six-minute window to intercept and destroy it. The radar system is still part of Cavalier Air Force Station.