A piece of uranium ore. Petr David Josek/AP

In 1942, physicist Enrico Fermi and a team of workers built what they thought was the first nuclear reactor in a Chicago racket ball court.

Unfortunately, nature had beaten them to the punch — by eons.

In Gabon, Africa, a spontaneous nuclear chain reaction started in a collection of uranium deposits about 2 billion years ago and kept going for hundreds of thousands of years.

According to Scientific American, these uranium deposits first caught the attention of nuclear scientists in 1972. That's when a worker analyzing the ore from the Oklo deposits noticed that something was off about his samples: The concentration of isotopes, or varieties of the same element with different neutron counts, was off.

The samples from Oklo were missing about 0.003% of uranium-235, the most precious of three isotopes present in natural uranium deposits.

That may sound like a tiny difference, but based on the size of the ore seam, it added up to 441 pounds of missing U-235. Since the isotope is rare yet used in both power plants and nuclear weapons, it's by far the most valuable version of uranium, and scientists wanted to know where it had gone.

It turns out it had been broken down during ancient chain reactions in a natural, self-sustaining nuclear reactor — the first ever discovered. Eventually, 17 natural reactor sites were found near the Oklo and Okelobondo uranium mines.

So how did it work?

Nuclear fission occurs when a neutron strikes an fissile isotope, breaking it apart and releasing more neutrons, propelled by the energy of the atomic split. The neutrons then hit other atoms, which break apart, and the reaction continues.

When uranium-235 decays naturally, one of its byproducts is a free neutron, which is why it could spur the reaction in the ore deposit.

For a nuclear reaction to be self-sustaining, however, it has to be surrounded by a moderator: a material that increases the likelihood of free neutrons smacking into the next atom and continuing the reaction. But it can't be surrounded by too many materials that would absorb the extra neutrons and grind the process to a halt.

Like in most modern-day nuclear power plants, the moderator in the Oklo deposits was water. Groundwater would seep into the deposit, boil away when the reaction got too hot, and temporarily shut everything down — but when the ground cooled and water returned to the reactor, the nuclear reaction would start up again.

Nuclear reactor zones (1) in Oklo were created by porous sandstone (2) allowing water to trickle into a seam of uranium ore (3) atop a section of granite (4). MesserWoland/Wikipedia (CC BY-SA 3.0)

These major pulses probably lasted hundreds of thousands of years, according to the scientists studying the site.

That is, until the reactor had split so much of its uranium-235 that the reaction had no fuel to continue. Then, millennia before humans stood on two legs, built physics labs, and split the atom, the reactor quietly shut down.

Scientists still aren't certain how many sites like those at Oklo exist in the world.

Perhaps one of the most fascinating things is what Oklo can teach the nuclear age about the disposal of nuclear waste: It's the closest thing we have to a long-term study of a nuclear waste disposal site.