In 1942, the US Army purchased 70 km2 near Denver, Colorado, to construct the Rocky Mountain Arsenal, where they manufactured chemical and incendiary weapons (mustard, lewisite, napalm) in support of the war effort. After World War II, Shell Oil and its predecessors manufactured pesticides, insecticides, and herbicides at the Arsenal, and the Army manufactured and then decommissioned Sarin (GB) nerve agent. The industrial manufacturing was concentrated near the center of the site, with many square kilometers of undeveloped land providing a buffer from urban Denver. Millions of liters of liquid wastes, including cyclodiene pesticides, such as dieldrin, aldrin, and endrin, were disposed of in open basins, pits, and trenches on the site. Cyclodiene pesticides impact the central nervous system of exposed biota, causing disorientation, emaciation, and eventually death. Thousands of wildlife mortalities were documented at the site, including an estimate of 20,000 duck deaths over a 10-year period in the 1950s, and over 1,800 waterfowl deaths in one basin alone between 1981 and 1987. After manufacturing ceased in the 1980s, the Army, Shell, the US Environmental Protection Agency, and the State of Colorado endeavored to address the contamination and the Arsenal’s future land use. In 1992, the USA passed the Rocky Mountain Arsenal National Wildlife Refuge Act to create wildlife habitat from uncontaminated and remediated areas and reduce risk of exposure to humans. Many aspects of the cleanup were based on pesticide risks to wildlife, and the parties rarely reached agreement on cleanup thresholds and the amount of area requiring remediation. Ultimately, in 2010, after nearly 30 years of investigation and cleanup efforts and over $2 billion, “significant environmental cleanup” was completed. Millions of metric tons of toxic sludges and soils had been placed in hazardous waste landfills, and millions of metric tons more were buried in place and capped. The refuge now comprises approximately 6,000 ha, roughly the size of Manhattan. The cleanup process was contentious, resulting in multiple lawsuits, which is not a model that the authors recommend. Although contamination at the Arsenal likely is still present and should continue to be evaluated, the refuge will be an important protected oasis of wildlife habitat in the midst of the Denver urban sprawl.