Oklo Fossil Reactors
In 1972 in Oklo, Gabon, while mining for uranium a very strange thing was discovered: the fossilized remains of a naturally occuring nuclear reactor. Later several other similar reactors were discovered, each dating back about two billion years. This is one of the few sites I've been able to find with explanations of the phenomenon and photographs of the site.
dead link: http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/