December 2, 1942, was the coldest day in Chicago in almost fifty years. That frigid afternoon, a crew of men and women—many of them hailing from countries an ocean away, where the Second World War raged—gathered under the viewing stands of the University of Chicago’s Stagg Field to light a secret fire. They were members of the Metallurgical Laboratory, an organization that had existed only since that January, and were attending to their creation, a dusty collection of graphite, uranium, and scientific equipment that they called the Pile. Today, we know it as something different: the world’s first nuclear reactor.

The Chicago Pile deserved its low-tech name. It was a stack of forty thousand graphite blocks, held together in a wooden frame, twenty-five feet wide and twenty feet tall. Inside about half of the blocks were holes containing small amounts of uranium oxide; inside a few others were nuggets of refined uranium metal, the production of which was still a novel process. The Pile had few safety features. The scientists’ only protection against radiation came from a set of cadmium control rods, designed to be inserted and removed by hand, along with untested theories and calculations. As one governmental report later put it, “there were no guidelines to follow and no previous knowledge to incorporate.” Neither university nor city officials were told that an experiment that even its creators judged as risky was taking place in the heart of the second-largest city in the United States.

The experiment itself was something of an anticlimax. The Pile was started up, brought to criticality (the point at which a nuclear reaction becomes self-sustaining), then shut down half an hour later, before its growing heat and radioactivity became too dangerous. The Metallurgical Laboratory experimented with it for a few months before disassembling and reconstituting it—now with radioactive shielding—at a site somewhat more removed from the city, where it became known as Chicago Pile-2. Ultimately, the reactor ran for over a decade before it was finally dismantled and buried in the woods.

The Pile was not an abstract scientific achievement. It was part of a much larger plan, conceived under the auspices of the Manhattan Project, to build a fleet of industrial-sized nuclear reactors—not for the generation of electrical power (that would come much later) but to produce plutonium, a fuel for nuclear weapons. Virtually overnight, the University of Chicago had become a major wartime contractor. (One of its many government contracts, by itself, doubled the school’s budget.) Data from the Pile would inform the design of later reactors, including the one that furnished the plutonium for history’s first nuclear-weapons test, known as Trinity, and the atomic bomb dropped on Nagasaki.

Wartime secrecy and suspicion suffused every aspect of the Metallurgical Laboratory’s work. The U.S. military had deemed some of its staff, including Arthur Compton, its Nobel Prize-winning director, security risks. Other members of the project, including the gadfly physicist Leo Szilard and even the eminent Enrico Fermi, were considered “enemy aliens,” because the countries from which they had fled were under Fascist rule. Vannevar Bush, the scientist-administrator who coördinated much of the early work on the Manhattan Project, appealed to the military to let these concerns slide. Rather than letting nuclear experts roam free, wouldn’t it be better, he suggested, “to take in and put under thorough control practically every physicist in the country having background knowledge of the subject”?

Eventually, the government addressed its security concerns by opening a new facility in a more isolated location, where the truly sensitive work could be done. This became the Los Alamos laboratory, in New Mexico. Though many of the Chicago team’s most trusted scientists made the journey to Los Alamos, others stayed—or were kept—behind. They did not, however, remain idle. Having completed the majority of their jobs in the early part of the Manhattan Project, and unburdened by the challenges of actually building the bomb, they had time to reflect on the social and political problems posed by the new technology. A report on this topic, chaired by James Franck, a Nobel Prize-winning physicist from Germany who had worked on chemical weapons in the previous war, concluded somewhat heretically that the first atomic weapons should not be dropped on cities without warning. The Franck Report elicited some discussion at higher levels of the Manhattan Project, but no plans were changed on account of it. Eventually, after the war, it was released to the public, with some alterations made by the military. One line that was scratched out of every copy of the report, but is just visible in originals by holding it up to the light at the right angle, argued that, should the United States be the first country to use nuclear weapons in war, it “might cause other nations to regard us as a nascent Germany.”

Not all of the Chicago scientists’ thoughts were so dark. Members of the Metallurgical Laboratory also wrote reports about the peaceful benefits of the atom, imagining a new field of science and technology, which they dubbed “nucleonics,” ushering in medical breakthroughs and new energy supplies in the wake of the Second World War. They recommended the creation of a national-laboratory system, to insure that organizations such as the Metallurgical Laboratory could exist in peacetime, and lobbied vigorously for what they considered wise policy on atomic weapons. The Bulletin of the Atomic Scientists of Chicago and the Federation of Atomic Scientists (later the Federation of American Scientists) both emerged out of this political awakening, and a movement for social responsibility on behalf of scientists was born. The Pile team turned out to be better at building reactors that changing public policy, but its legacy of activism and public engagement reverberates in today’s discourse about climate change.

After the war had ended and the world had come to appreciate the power that had been unleashed, the University of Chicago installed a bronze plaque commemorating the Pile. It read, “On December 2, 1942, man achieved here the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy.” In a rejected suggestion, the university press director proposed that a phrase be added to the end: “for better or worse.”