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The footprint of the former K-25 building, which enriched uranium for America's nuclear weapons, is seen in Oak Ridge, Tennessee, USA, 11 June 2015. Built in secret during the Manhattan Project, the U-shaped K-25 was once the largest building in the world under one roof. Twelve thousands employees worked inside; uranium enriched at the plant was in the atomic bomb that the US dropped on Hiroshima, Japan. The Department of Energy completed a five-year-long demolition of the massive building in 2014.