Oklo natural reactor

About two billion years ago, in a uranium deposit in what is now Gabon, nuclear fission with a chain reaction started, resulting in criticality. 

The necessary conditions for such a natural reactor were prevailing In Mounana, which lies in today's Haute-Ogoueé province. Here, the amount and concentration of uranium-235 were sufficient for a chain reaction, there were no neutron-absorbing materials such as cadmium and lead, and natural water served as a moderator.

The chain reaction came to a halt at least about 1.5 billion years ago, when the uranium-235 was used up far enough.