On April 7,1972, scientists at the French Atomic Energy Establishment at Pierrelatte discovered a 0.4% anomaly in the uranium isotopic ratio of a sample of natural uranium. In a masterful example of analytical detective work, the French scientists traced the anomaly to a particular area of a uranium mine located at Oklo in the Republic of Gabon. In that area they found uranium that was deficient in U-235 and was accompanied by fission-produced isotopes of Nd and Sm. On September 25, 1972, the French Atomic Energy Commission announced their conclusion that self-sustaining nuclear chain reactions had occurred on Earth about 2000 million years ago [Neuilly et al., 1972]. Later, other natural nuclear fission reactors were discovered in the region [Gauthier-Lafaye et al., 1996].