Westward view of Rocky Flats Nuclear Facility, built northwest of Denver by the US government in 1951 to produce nuclear bomb cores. Rocky Flats operated in this capacity until it was decommissioned  in 1991 following the end of the Cold War. During the 1980s it was found that toxic waste from the bomb core production process had been contaminating the surrounding environment for decades; cleanup began in the 1990s, and in 2007 the US Fish and Wildlife Service acquired the site, turning it into a national wildlife refuge.