Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor. CP-1 was built on a racquets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first artificial, self-sustaining, nuclear chain reaction was initiated within CP-1, on December 2, 1942.
I used to play tennis at the place where the racquets courts used to be. That would have been in 1962. There was a nice plaque commemorating the event attached to the tennis court fences. It is now suitably mounted on a small monument about 100 ft South of where the tennis courts used to be.