Shown: The discovery of a 2-billion-year-old naturally-occurring nuclear reactor in Oklo, Gabon, Africa. What happened was that a uranium deposit in Precambrian times was triggered by groundwater seeping through sandstone. A chain reaction would initiate, boiling the water away, which would then cool the reaction but allow more water to seep in and repeat the cycle. The process apparently kept going for hundreds of thousands of years,
Oklo is the site of the only known natural nuclear reaction. French physicist Francis Perrin discovered the natural phenomenon in 1972. The French mined out the uranium in the area until supplies were depleted, but geologists and physicists have continued to study the area for clues about what happened there millenniums ago.
A great little educational video on how nuclear energy generators work: