Remnants of the Oklo Naturual Nuclear Reactor, Gabon (West Africa)
Not all nuclear reactors on Earth are manmade — some arise naturally. The Oklo mine in the West African nation of Gabon was once home to a natural nuclear reactor that apparently spontaneously turned on 2 billion years ago, running on uranium fuel. It seems to have lasted about 150,000 years with an average power output of 100 kilowatts, radiating the energy equivalent of 100 megaton bombs over its lifetime.
(via: Live Science) (image: Robert D. Loss, WAISRC)