In the 1940s, a huge chunk of land in the industrial part of Denver, Colorado was used as a chemical weapon manufacturing site for the war. The Rocky Mountain Arsenal (RMA) continued its industrial operations wherein waste disposal practices led to contamination of natural resources.

All industrial activities ceased in 1982. In 1987, RMA was placed on the National Priorities List, a collection of U.S. toxic waste sites that required long-term cleanup to be funded by the federal government's Superfund program.

In the course of environmental cleanup, investigators found bald eagles, which use to be an endangered species, on the site. Further research from the U.S. Fish and Wildlife Service (USFWS) revealed the site had become home to over 330 wildlife species over time. In October 1992, RMA became the Rocky Mountain Arsenal National Wildlife Refuge under the Bush administration. In 2010, the $2.1-billion environmental cleanup was finally completed.

"This remarkable place shows that nature will recover and will thrive if given a chance," said Dan Ashe, USFWS Director who witnessed the release of the black-footed ferrets.

On Oct. 5, USFWS released around 30 rare black-footed ferrets on the 25-square-mile refuge site. Environmentalists thought the species were extinct. In 1981, a small group was discovered in Wyoming which prompted wildlife researchers to grow its population since.

An adult ferret's long and slender body can grow up to 24 inches long, despite having short legs. Sometimes called the 'black-eyed ferret', these species have a distinct 'black mask' on the face. With an average weight of 2.5 pounds, black-footed ferrets are hairy. They also have bad teeth and a bad-boy attitude, said outreach specialist Kimberly Fraser who joined the ferret release program. Nocturnal by nature, the species spend almost 90 percent of the day underground. They normally go out at night to hunt for food.

Black-footed ferrets prey on prairie dogs, whose declining population affected the former's survival. Prairie dogs usually die of pest control and plaque. The decreased number of prairie dogs led to the decline of black-footed ferret's population in in eastern Nebraska to western Arizona, as well as southern Canada to northern Mexico.

The Black-Footed Ferret Recovery Program was created in 1996 to help restore the species' population. The program manages six facilities where they breed captive animals and train them to hunt prairie dogs outdoors.

USFWS planned to return and reintroduce these black-footed ferrets to their natural habitat. To date, the species have been reestablished in 24 sites including Montana, South Dakota, New Mexico, Wyoming, Utah, Kansas, Colorado and Arizona. Reintroduction outside the U.S. border took place in Chihuahua, Mexico and Saskatchewan, Canada. Program coordinator Pete Gober expects a survival rate of up to 75 percent, as long as prairie dogs don't die of plague.

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