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In 1972 in Oklo, Gabon, a uranium mine was being operated. But when was taken to be enriched, it was discovered that the U-235 to U-238 ratio was very odd. A careful examination of the mine turned up fossil evidence of several naturally occuring nuclear fission reactors. At least one of them was a fully functional breeder reactor: from the fissioning U-235 a neutron converts U-238 to U-239 which beta decays to Pu-239, and then that alpha decays to a new U-235. There is not much left to these reactors due to the mining, but there are a few remain, and number fifteen is typically described as “well preserved.” Here are a one page summary of the Oklo reactors and a more detailed discussion of Oklo. These natural reactors are not well known to the public, but well studied for insights into long term nuclear waste storage.