It’s been almost 25 years since the U.S. Department of Energy, the Environmental Protection Agency, and the Washington Department of Ecology entered into an agreement to clean up the Hanford Site — widely considered to be the most contaminated nuclear site in the nation.

The 586-square-mile site located on the Columbia River north of Washington’s Tri-Cities was responsible for producing most of the plutonium for the U.S. nuclear weapons program during the Cold War, as well as hundreds of billions of gallons of hazardous waste before production ended and cleanup began in 1989.