Oldest nuclear chain reaction

On 2 June 1972, a French scientist analysing uranium ore samples in Gabon discovered tiny anomilies in the ratio of isotopes. This variation from the norm led to the discovery of the Okla natural fossil reactors. Around 2,000,000,000 years ago, these were active underground nuclear fission reactors, powered by natural uranium ore which had been concentrated by geological processes, and kept cool by the flow of natural groundwater. Sixteen of these sites have now been identified in the Okla uranium deposit, and one other in the deposit at Bangombe. Both locations are in Gabon. So much time has passed since the reactors were active that the 'waste' is no longer radioactive.