Earth built nuclear reactors before humans

Around 2 billion years ago, uranium deposits at Oklo, in what is now Gabon, sustained natural fission reactions, long before humans existed. Concentrated uranium, the right rock and groundwater came together naturally; the water slowed neutrons, helping the chain reactions continue. The evidence was discovered in 1972, when scientists noticed unusual proportions of uranium isotopes and found traces left by fission. Surviving rock samples preserve the story of these natural reactors.

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