Oklo, traces of a natural reactor
This piece of cliff on the site of the Oklo uranium mines in Gabon contains traces of the operation of a fossil reactor, discovered during the 1970s. Sixteen 'natural' reactors were in operation at Oklo for several hundreds of thousands of years, two billion years ago. A conjunction of favourable factors, including in particular its content of fissile uranium-235, which was much higher then than it is now, contributed to this extraordinary phenomenon.