Chicago Pile-1, built secretly beneath Stagg Field at the University of Chicago, became the world’s first artificial nuclear reactor when Enrico Fermi’s team achieved the first human-made self-sustaining nuclear chain reaction on 2 December 1942. Using ultra-pure graphite and uranium, the reactor ran at very low power but proved that a controlled chain reaction was possible. The breakthrough grew from years of work by Fermi, Leo Szilard, and others, including solving the impurity problems that had doomed earlier graphite tests. Its success gave the Manhattan Project a practical foundation for reactor research and plutonium production, making the site one of the twentieth century’s pivotal scientific landmarks.