http://www.atlasobscura.com/places/oklo-reactor
'' the uranium deposits in the Oklo region of Gabon created a natural nuclear power plant that operated for hundreds of thousands of years until most of the fissile uranium was depleted. While a majority of the uranium at Oklo is the non-fissile isotope U238, only about 3% needed to be the fissile isotope U235 for the chain reaction to start. Today, that percent of fissile uranium in the deposits is around 0.7%, indicating that the deposit had sustained reactions for a relatively long period of time. But it was this exact characteristic of the rocks from Oklo that first puzzled scientists''
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