Rocky Mountain Arsenal, Colorado

27 square miles of what was once considered by many to have been "the most toxic land in America." The sprawling plant was established in WWII to manufacture chemical weapons such as mustard gas and napalm. It was later taken over by private companies, including the Shell Chemical Company, and was used in the manufacture of explosives and insecticides until the early 1980s, when contamination issues took precedence over production (in the 1970s, for example, live chemical weapons projectiles were found in dirt that had been bulldozed to make a parking lot). In 1987, the EPA declared it an official Superfund cleanup site. Remediation efforts on the part of both the U.S. Army and Shell Oil Company, were completed in 2010. The site is now a 15,000-acre national wildlife refuge, although the U.S. Army has retained 1,000 acres containing landfills, waste collection areas, and groundwater treatment facilities, which they will continue to control and manage.