(Wikipedia) On December 2, 1942, under the direction of the brilliant Enrico Fermi, the world’s first self-sustaining nuclear chain reaction took place in the world’s first nuclear reactor, Chicago Pile-1, shortened as CP-1, built on a racquets court under the abandoned west stands of the Alonzo Stagg Field stadium on the University of Chicago campus. Operation of CP-1 was terminated in February 1943 and the nuclear reactor dismantled and moved to the laboratory’s Palos Park site A. It was reconstructed using CP-1 materials but enlarged with a radiation shield and named CP-2. It began operation in March 1943. More….
Note: the stadium was torn down in 1957.