A prehistoric natural “nuclear reactor”, which was in operation about 2 bn years ago, was discovered in the uranium deposit Oklo/Gabon in 1972. In past years, six further locations in this deposit were found where a self-perpetuating chain reaction must have taken place due to the reduced U-235 content in the natural uranium. For the Oklo II location, it can be calculated from the depletion of uranium-235 entailed by fission that a minimum of 4 t U-235 must have been fissioned, 1 t Pu-239 formed, and a volume of heat of about 100 bn kWh generated. As a comparison: in the reactor of a nuclear power plant of the 1,300 MWe category, about 30 bn kWh heat is generated by fission annually.