Oak Ridge K-25 Building Site, Tennessee

Oak Ridge National Lab was established during the Manhattan Project to produce nuclear fuel for the first atomic bombs, and continued this and related functions into the 1990s, when its mission transformed more into closure and environmental remediation of its facilities. One of the principal complexes at Oak Ridge was the K-25 area, finished in a great hurry in 1944 to enrich uranium through gaseous diffusion. With 1,640,000 square feet of floor space the main plant was the largest building in the nation for some time. It was in use until 1985, and mostly torn down in 2013. Two other gaseous diffusion plants were constructed in the 1950s, at Paducah, Kentucky, and Piketon, Ohio.