 Much of this highly radioactive material falls to earth, subjecting anything within the line of sight to radiation, becoming a significant hazard.A fallout shelter is designed to allow its occupants to minimize exposure to harmful fallout until radioactivity has decayed to a safer level.During the Cold War, many countries built fallout shelters for high-ranking government officials and crucial military facilities, such as Project Greek Island and the Cheyenne Mountain nuclear bunker in the United States and Canada's Emergency Government Headquarters.