The world's first nuclear chain reaction took place 75 years ago today. On December 2, 1942, physicists Enrico Fermi and Leo Szilard led the team that conducted the experiment at the University of Chicago as part of the Manhattan Project. The possibility of such a chain reaction was discovered in 1938, but there were some kinks to work out.
For a sustained, controlled chain reaction, each fission must induce just one additional fission. Any more, and there’d be an explosion. Any fewer and the reaction would peter out.
The words "there'd be an explosion" sounds like quite the understatement, considering how the Manhattan Project turned out. But the chain reaction achieved that day also led to the development of nuclear energy plants.
Read an outline of the science behind the first nuclear chain reaction at Smithsonian.