oklo

In the early- to mid-1950s, Dr. Paul Kuroda from the University of Arkansas described the possibility of naturally occurring nuclear reactors lurking in the crust of ancient Earth... Such a reactor could not exist today, because too much of the Earth's natural U-235 has decayed… but a billion and a half years ago, there was enough of it around to make the idea plausible. In point of fact, it has since been discovered that it actually happened. Damn Interesting, Wikipedia.