Naturally occurring nuclear fission reactors that are believed to have existed in uranium deposits at Oklo in Gabon, West Africa, about 2000 million years ago. In 1972, French scientists noticed a slight difference in the normal 235U/238U ratio in uranium ore from Oklo. Further detailed investigations showed that there had been 15 natural reactors in the ore deposits at Oklo, operating intermittently for about 1 million years. It is thought that the geology of the mine was an important factor in the creation of these reactors, in particular, the seepage of water through overlying rock, which functioned as a moderator. A similar natural reactor has been found at Bangombe, some miles south of Oklo, but no other comparable reactors have been found anywhere in the world. The Oklo reactors are of considerable interest. They involve basic nuclear processes occurring 2000 million years ago and might give insights into the time dependence of fundamental constants. More practically, Oklo can be regarded as a 2000-million-year experiment in the containment of nuclear waste. The reactors shut down naturally when the proportion of 235U decreased, and – for the same reason – natural reactors of this type could not occur today. The products of the reactor have, however, been localized because of the geology of the region, in particular, beds of granite underlying the ore deposits.