Case file · High strangeness
Oklo, the natural nuclear reactor
In 1972 French scientists found that uranium ore from Oklo in Gabon was short of the fissile isotope uranium-235, and traced the loss to natural nuclear chain reactions that had run there about 1.7 billion years ago.
1972 CE · -1.39, 13.16 · status: explained
The account
Groundwater seeping into rich uranium seams acted as a moderator, starting reactions that ran on and off for hundreds of thousands of years. Online, Oklo became evidence for a lost civilisation or alien reactor.
The sceptical view
Physicists predicted such reactors in 1956; Oklo is the only known example and is entirely natural.