Keeping with the nuclear power theme, apparently there were naturally-occurring nuclear fission reactors in Africa about 2 billion years ago. The 15 reactors were buried in what is now the Oklo uranium mine in southeast Gabon. They ran off of Uranium 235 (just like man-made nuclear reactors), generating 100 kilowatts for about 150,000 years. Groundwater evaporation and condensation kept them on a 3-hour cycle that prevented meltdowns. More recent research indicates a natural reactor probably occurred at Bangombé, about 22 miles away, around the same time.

While interesting merely as a phenomenon, scientists are more concerned with applying lessons learned from the natural reactors to the disposal of nuclear waste at places like Yucca Mountain. Visit the Department of Energy’s Office of Civilian Radioactive Waste Management for a detailed fact sheet.

Dr Robert Loss at the Curtin University of Technology in Australia
has also assembled an explanation but it has some dead links.

Link: "The Pulse of a Nuclear Reactor "
(American Physical Society’s Physical Review Focus)