Oklo was natural nuclear reactor about 1.8 billion years ago in Gabon, West Africa.
Thirteen nuclear reactors were discovered along a 200-metre uranium mine bed.
Through a quirk of nature, concentration of U-235 were high enough to permit nuclear fissions to occur.
Scientists think these natural reactors could have functioned intermittently for a million years. The average power output was 100 kilowatts. Over its entire running time, the reactor put out the energy equivalent of one hundred Megaton bombs. The natural chain reactions stopped when the uranium isotopes became too sparse to keep the reactions going.