In 1972, a pocket of uranium in Africa was found to have undergone self-sustaining nuclear fission for hundreds of thousands of years, making it the only known naturally formed nuclear reactor.
Many people think nuclear power is an invention of mankind, and some even think it violates the laws of nature. But nuclear power is a naturally occurring phenomenon, and life could not exist without it. That's because our sun (and every other star for that matter) is itself a giant power plant, lighting up the solar system through a process known as thermonuclear fusion.
Humans, however, generate power through a different process called nuclear fission, which releases energy by splitting atoms rather than combining them as in the fusion process. No matter how ingenious our race may seem though, even fission reactors are old news to Mother Nature. In a singular but well-documented circumstance, scientists have found evidence that naturally occurring fission reactors were created inside three uranium ore deposits in the West African country of Gabon.
Two billion years ago, the uranium-rich mineral deposits became flooded with groundwater, setting off a self-sustaining nuclear chain reaction. The energy that was subsequently released then raised temperatures enough to begin boiling away the water, but when the mineral deposits eventually cooled down the nuclear reactions would start back up again. By examining the levels of certain isotopes of xenon gas (a byproduct of the uranium fission process) in the surrounding rock, scientists have determined the natural reactor proceeded in this start-stop fashion at intervals of about two and a half hours.