Several naturally occurring nuclear reactors have been discovered in Gabon, Africa. Groundwater flooding deposits of uranium ore made the reaction possible.
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, short-lived fission product poisons decayed, the water returned and the reaction started again. These fission reactions were sustained for hundreds of thousands of years, until a chain reaction could no longer be supported. Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 2 hours and 30 minutes
Nice try Fermi, but Mother Nature got there first.
BTW, despite reading The Making of the Atomic Bomb (twice!), I can't recall hearing this pair of anecdotes before:
Due to a mistranslation, Soviet reports on Enrico Fermi claimed that his work was performed in a converted "pumpkin field" instead of a "squash court", squash being an offshoot of hard racquets.
When the first self-sustained nuclear chain reaction was achieved, a coded phone call was made by one of the physicists, Arthur Compton, to James Conant, chairman of the National Defense Research Committee. The conversation was in impromptu code:
Compton: The Italian navigator has landed in the New World.
Conant: How were the natives?
Compton: Very friendly.
Pumpkin field, tube alloy, the Italian navigator, the Manhattan Project...the building of the atomic bomb had no shortage of fanciful language.
Update: BLDGBLOG did a post on fossil reactors recently, which is probably where I got the link above in the first place.