A uranium deposit in Gabon ran as a nuclear reactor on its own 1.7 billion years ago
In 1972, French scientists noticed uranium ore from the Oklo mine in Gabon was oddly depleted in fissile uranium-235, the same signature left in spent reactor fuel. The cause was a natural chain reaction: 1.7 billion years ago, when U-235 made up a much higher share of natural uranium, groundwater seeping through a rich seam acted as a moderator and let the deposit sustain nuclear fission on and off for hundreds of thousands of years. It remains the only known natural nuclear reactor.