This is amazing! A natural nuclear reactor has been discovered in the African country of Gabon in 1972. It has only recently been determined how it worked.

From the article:

“[R]iver water trickling into uranium-rich bedrock acted like the control rods in a modern reactor, increasing the efficiency of fission and causing the uranium to produce a chain reaction. The reaction released heat that boiled the water. Once all the water was gone, the fission fizzled out, preventing a meltdown. Gradually, more water trickled in and the process started anew.”