This is an article on the history of the first controlled nuclear fission that was conducted at the University of Chicago 75 years ago that marked the beginning of the atomic/nuclear age.
They called this 20x6x25-foot setup Chicago Pile Number One, or CP-1 for short – and it was here they obtained world’s the first controlled nuclear chain reaction on December 2, 1942. A single random neutron was enough to start the chain reaction process once the physicists assembled CP-1. The first neutron would induce fission on a uranium nucleus, emitting a set of new neutrons. These secondary neutrons hit carbon nuclei in the graphite and slowed down. Then they’d run into other uranium nuclei and induce a second round of fission reactions, emit even more neutrons, and on and on. The cadmium control rods made sure the process wouldn’t continue indefinitely, because Fermi and his team could choose exactly how and where to insert them to control the chain reaction.
Sadly, other than a commemorative statue/plaque, there's not much left of this historic site. One of the outcome of this work is the creation of Argonne National Lab just outside of Chicago, where, I believe, the research on nuclear chain reaction continued at that time. Argonne now no longer carries any nuclear research work.