“This cannot be possible.”

That was the thought racing through physicist Francis Perrin’s mind in May 1972. He was examining a dark piece of uranium ore at a nuclear fuel processing plant in southern France. The uranium sample, extracted from a mine in Gabon, Africa, held a secret that defied everything scientists knew about natural uranium.

Normally, uranium consists of a consistent ratio of isotopes: uranium-238, uranium-234, and the coveted uranium-235. Across Earth’s crust, the ratio of uranium-235 is steady at 0.720%. Yet, this sample from Gabon contained only 0.717%. A tiny deviation — but enough to set off alarms. The simplest explanation would be that the uranium had undergone fission. But how could that be, since this was a natural sample?

Had someone tampered with the uranium? Was this an artifact of an ancient, unknown civilization? Or, perhaps, something even stranger?

Fission in the Wild

The more the scientists investigated, the stranger it got. In some uranium samples from Gabon’s Oklo region, the uranium-235 ratio was even lower, dipping to 0.4%. That was more than a statistical fluke — it meant something profound had altered the ore.

Further analysis revealed that the uranium had indeed undergone fission, the same process used in nuclear reactors. But this wasn’t the result of human interference, nor an alien species. The evidence pointed to an event that took place two billion years ago.

Suddenly, the unthinkable became clear: nature had created its own nuclear reactor.

For 40 years, France mined uranium in Gabon, which was then its colony. France is a nuclear powerhouse and used the uranium to produce electricity, both in France and elsewhere Europe. When uranium deposits were discovered near the town of Oklo, Gabon, it was exciting news, but no one really understood what they’d discovered; not at first, at least.

That’s how Perrin ended up analyzing the strange sample. He and his colleagues confirmed that it’s a natural sample that had undergone fusion back when Earth was still a young planet.

Man-made nuclear fission reactors work by carefully controlling a chain reaction where uranium-235 atoms are split by neutrons, releasing energy and more neutrons, which go on to split additional atoms. To sustain this reaction, enriched uranium—where the concentration of uranium-235 is increased. The creation of such reactors requires advanced technology, precise engineering, and rigorous safety protocols. It’s not something that you’d expect to just pop up in nature.

Yet, in Oklo, billions of years ago, nature spontaneously provided the right combination of uranium concentration, water, and geological stability to sustain a controlled fission reaction.

How to form a natural reactor

In 1956, a chemist called Paul K. Kuroda predicted that under the right circumstances, natural fission reactors could form. His work received some attention, but it wasn’t an instant hit as the conditions seemed very unlikely, and few people (if any) truly expected to find something like this.

Kuroda estimated that the uranium deposit needs to be at least 0.66 meters (approximately two feet) to sustain a natural nuclear fission reaction. If the deposit is smaller than this, it can’t reach critical mass. Then, you need to have enough uranium-235 in the deposit.

Two billion years ago, uranium-235 was far more abundant than it is today. Back then, it made up about 3 percent of natural uranium — similar to the enriched uranium used in modern nuclear reactors. Just like modern reactors, the process needed a moderator to slow down neutrons and make fission more likely. At Oklo, groundwater played this crucial role. The water slowed the neutrons, facilitating a sustained chain reaction.

“Like in a man-made light-water nuclear reactor, the fission reactions, without anything to slow down the neutrons, to moderate them, simply stop,” said Peter Woods, team leader in charge of uranium production at the International Atomic Energy Aassociation. “The water acted in Oklo as a moderator, absorbing the neutrons, controlling the chain reaction.”

Elements like boron or lithium, which absorb neutrons and halt fission, needed to be absent. Fortunately, Oklo’s deposits lacked these “contaminants,” allowing the reaction to continue. When these conditions aligned perfectly, the result was a natural nuclear reactor.

The ancient reactor at Oklo didn’t run continuously. Researchers dated the rocks and analyzed past activity. So, they found that the Oko reactor operated in cycles.

As groundwater infiltrated the uranium deposits, it moderated the neutrons, allowing fission to occur. The reaction heated the water, which eventually boiled off as steam. Without the water to moderate the neutrons, the reaction would stop.

“That’s what makes it so fascinating: that the circumstances of time, geology, water came together for this to happen at all,” Woods said. “And that it was preserved until today. The detective story has been successfully solved.”

After the area cooled and more groundwater seeped in, the reaction would start again. Then, this cycle repeated for hundreds of thousands of years. A study on the Oklo reactor concluded:

“About 15,000 megawatt-years of fission energy was produced, over a period of several hundred thousand years, equivalent to the operation of a large 1,500-MW reactor for ten years.”

A natural site like no other

News of this natural phenomenon spread quickly. In 1975, physicists from around the world gathered in Libreville, Gabon, to discuss what became known as the Oklo Phenomenon. The discovery was stunning. Nature, it turned out, had mastered nuclear power long before humans ever imagined it.

Yet, although some theoretical predictions fit with the observations, it wasn’t easy to prove what was going on. There were four spots that operated as natural, all in the same geological structures.

The key to solving these puzzles lay in an unlikely source: xenon gas. This inert gas, trapped in minerals at Oklo, acted as a time capsule.

Different xenon isotopes form during nuclear fission, and their ratios can reveal the conditions under which fission occurred. Alex P. Meshik, a physicist, studied these xenon isotopes and found that they held clues to the reactor’s stability. The trapped xenon showed that the reactor’s fission reactions were remarkably stable, cycling on and off as groundwater levels changed.

The xenon also revealed how the reactor finally shut down. Over time, the uranium-235 was gradually consumed, reducing the fuel supply below the critical threshold needed to sustain fission.

Today, the uranium mines of Oklo are exhausted, but the legacy of the world’s only known natural nuclear reactors lives on. Samples of the Oklo reactors are preserved in museums, like Vienna’s Natural History Museum, where visitors can see rocks produced by nature’s fission reactor.

There may be other natural reactors as well, we just haven’t found them yet. In the meantime, humankind is focusing on its own fission reactors more than on nature’s.