An ancient nuclear reactor that was churning away 2 billion years ago sounds like a myth. Maybe it's because the term reactor usually implies a manmade structure. But in this case, the reactor is a region of natural uranium within the Earth's crust, found in Okla, Gabon. Uranium is naturally radioactive, and the conditions in this rocky area happened to be just right to cook up some nuclear reactions.
The natural reactor must have annoyed nuclear scientists: The first nuclear reactor to produce electricity started up in 1951, and this only produced a trivial amount of energy. The pile of rock in the ground in Okla, on the other hand, created nuclear power around 2 billion years ago!
It was discovered in 1972, when some French scientists took uranium ore from the mine in Gabon to test its uranium content. Typically, uranium ore is made up of three types (isotopes) of uranium, each one with a different number of neutrons: Uranium 238, which is the most abundant, uranium 234, which is the rarest, and uranium 235, which nuclear scientists are most interested in because it can sustain nuclear chain reactions.
One would expect to find that the uranium ore is composed of 0.720% uranium 235, since that is the percentage found in other rock samples from the Earth's crust, the Moon and even meteorites. However, these French scientists found something fishy: The uranium sample only contained 0.717% of uranium 235. What might seem to be a minor discrepancy of 0.003% is very significant with regards to uranium.