In 1972 a French engineer, named Bougzigues, discovered the Oklo natural reactors. Bougzigues worked as an analyst for the Pierrelatte nuclear fuel processing plant which produced fuel for nuclear power plants in Provance, France. During routine mass spectrometry measurements of the value of 235U/238U ratio in U ore samples he observed a tiny change in the isotope ratio (0.00717, compared to a normal value of 0.00720). So precisely known is this ratio that this small difference was sufficient to suggest something strange had occurred.
At first it was thought that some used nuclear fuel had inadvertently slipped into the processing plant. However, this was quickly ruled out because of the lack of intense radiation normally found in spent fuel.
Other possibilities raised included spent fuel from an extra-terrestrial space ship or uranium from an ancient nuclear waste site perhaps even from a past civilization. A careful check on the source materials traced the uranium ore back to a very high concentration uranium deposit present in a mine site in Gabon, a country in South West Africa.
A detailed investigation detected the presence of all the conditions necessary for, and large quantities of ancient (no longer radioactive) fission product waste embedded in the natural uranium ore, confirming that natural nuclear fission reactions had taken place at Oklo some 2000 million years ago.
Read more at WikiNuclear. [I should point out that Gabon is in West Africa rather than South West Africa.]