You're looking at the only photo that was taken of the construction of the first man-made nuclear reactor. It was called Chicago Pile-1 (or CP-1), and was put together by Enrico Fermi and his students underneath the bleachers of an abandoned football stadium. The reason it was called a "pile" is that it was a carefully constructed, but still literal pile, of graphite bricks and uranium pellets. Control rods moderated the reaction, but there was no radiation shielding, and no cooling system, besides three guys standing, ready to douse the reactor with water if things got out of control. CP-1's first run was 28 minutes long, and it didn't kill anyone, and after some additional testing, it was buried at Red Gate Woods, where you can go visit it if you want.
Incidentally, the reason we've got "man-made" in the headline is that there are much much older natural nuclear reactors producing energy underground on Earth as early as two billion years ago. These reactors formed in uranium ore deposits moderated by intermittent groundwater, and probably produced an average of 100 kilowatts of thermal energy on and off for a few hundred thousand years. Hot stuff, and it only took humans a couple billion years to catch up.
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