At the end of 1938 in Berlin, Otto Hahn and Fritz Strassmann observed that uranium nuclei split in two under neutron bombardment; a few months later Lise Meitner and Otto Frisch interpreted the phenomenon and called it "fission." Each split released more neutrons than it absorbed — so if those neutrons could go on splitting other uranium nuclei, a self-sustaining "chain reaction" was possible. The practical question was: how do you hold and steer the reaction? An uncontrolled chain is a bomb; a controlled chain is an energy source.

The Manhattan Project's metallurgical laboratory was organised at the University of Chicago. The Italian émigré physicist Enrico Fermi took charge of pile design; Leo Szilárd had been arguing for the chain-reaction idea since 1933. The design looked plain but was vast: about 350 tons of graphite blocks, with 5.4 tons of uranium metal and 45 tons of uranium oxide embedded in it. Graphite acted as a "moderator," slowing the neutrons and increasing the chance that the next nucleus would capture them. Cadmium-coated control rods hung down to quench the pile; a so-called "suicide squad" stood ready on a platform to dump buckets of cadmium sulphate solution onto it in emergency.

On the morning of 2 December 1942, beneath the west stands of Stagg Field on a racquets court, the team brought the pile up slowly. The control rods were withdrawn centimetre by centimetre and the neutron counters "click click click" sped up. At 3:25 p.m. Fermi said softly, "The pile has gone critical." The reaction ran in a controlled state for 28 minutes and was then shut down. Total power output was about half a watt — not enough to light a torch, but proof of the phenomenon. Eugene Wigner opened a bottle of Chianti; everyone drank from paper cups and signed the bottle. Compton phoned the news in code to Harvard's president Conant: "The Italian navigator has landed in the New World."

The door opened both ways at once. The logic of CP-1 was scaled up three years later into Hanford's giant plutonium-producing reactors; the plutonium they made formed the core of the Nagasaki bomb. The same logic became, in 1956, the first commercial nuclear power station feeding the British grid at Calder Hall. Today roughly a tenth of the world's electricity is nuclear — low in greenhouse emissions but, as Chernobyl and Fukushima have shown, a technology whose accidents leave radioactive waste at a regional scale. The site where CP-1 ran was dismantled in 1957, its waste buried, and Henry Moore's "Nuclear Energy" sculpture set above it — a mass swaying between a skull and a mushroom cloud. Even as it is memorialised, the event carries its double meaning: humans took into their own hands, for the first time, one of the forces that runs inside the stars; the same act handed them the technique of their own destruction.