Stagg Field sits on the northwest corner of the University of Chicago’s Hyde Park campus. These days, it’s home to a baseball field, tennis courts, a track and a football field—pretty standard for a major university. But on November 16, 1942, in an old squash court beneath a set of bleachers, workers began building Chicago Pile-1: the world’s first working nuclear reactor.

The experimental reactor was built during the height of World War II as part of the Manhattan Project, the army’s nuclear weapons program. Led by physicist Enrico Fermi, who described the rudimentary reactor as "a crude pile of black bricks and wooden timbers," CP-1 was built in a matter of weeks out of a large stack of graphite bricks and uranium pellets, with cadmium and iridium control rods inserted to keep it from going critical, Michael Byrne writes for Motherboard.

Fermi theorized that the uranium would act as fuel by emitting neutrons that would collide with the other uranium atoms in the pile and split them apart. The more atoms that split, the more energy they would release, which would in turn perpetuate the reaction. The graphite bricks would slow the uranium neutrons, making these collisions more likely; control rods absorbed the neutrons, allowing Fermi and his team to control the reaction.

CP-1 was completed on December 1, and the control rods were removed the next day. Within hours, the reactor went critical in the first sustained artificial nuclear reaction, Byrne reports.

The old squash courts weren’t Fermi’s first choice: CP-1 was supposed to be built in the Red Gate Woods southwest of the city, but workers at the site were on strike. Faced with the choice of cancelling the experiment or conducting it at Stagg Field, Fermi convinced the project’s overseer, physicist Arthur Compton, that the reactor was safe enough to build under the bleachers, Katie Mingle reported for WBEZ's Curious City.

Even though Stagg Field wasn’t used much at the time, CP-1 lacked radiation shielding to protect workers or onlookers, and meltdown was a considerable risk. Luckily the experiment worked and the reactor was dismantled and relocated to Red Gate Woods shortly after.

The bleachers and the squash court no longer stand: Workers bulldozed the original Stagg Field rebuilt to make room for a new library. Where the bleachers once stood, a bronze statue stands to commemorate the experiment that kickstarted the atomic age. And deep in the Red Gate Woods, the pile sit buried under a field, marked with a simple gravestone that tells anyone who stumbles on it that they are walking above one of the most important artifacts of the 20th century.