Site A

Site A
	Laboratory buildings at Site A
Established	1942
Field of research	Nuclear physics
Location	41°42′08″N 87°54′44″W / 41.70222°N 87.91222°W
Operating agency	University of Chicago

Site A was a research facility near Chicago where, during World War II, research on behalf of the Manhattan Project was carried out. Operated by the University of Chicago's Metallurgical Laboratory, it was the site of Chicago Pile-2, a reconstructed and enlarged version of the world's first nuclear reactor, Chicago Pile-1. The first heavy-water reactor, Chicago Pile-3, was also constructed at this site. Research was carried out under contract to the United States' Office of Scientific Research and Development. After the war, the site became the first home of Argonne National Laboratory, a federally funded research and development center.

The site was returned to public use in 1956, but Site A, and a nearby site formerly used for the disposal of low-level radioactive waste, Plot M, continue to be managed by the Department of Energy's Office of Legacy Management as the Site A/Plot M Disposal Site.

The Site A/Plot M Disposal Site is located within Red Gate Woods in the Palos Forest Preserves, part of the Forest Preserve District of Cook County. The site contains buried radioactive waste from contaminated building debris, and the Chicago Pile-1 (CP-1/CP-2), and Chicago Pile-3 (CP-3) nuclear reactors. "Site A" was an early Manhattan Project code for the facility. "Plot M" was the code name used for the disposal ground.