Some of the most fascinating phenomena I've read about are the natural nuclear reactors that the French discovered in Oklo, Gabon, in 1972. A search on Google for "natural nuclear reactors oklo" yields hundreds of hits. Here are some highlights. From Yucca Mountain Project:
Deep under African soil, about 1.7 billion years ago, natural conditions prompted underground nuclear reactions. Scientists from around the world, including American scientists have studied the rocks at Oklo. These scientists believe that water filtering down through crevices in the rock played a key role. Without water, it would have been nearly impossible for natural reactors to sustain chain reactions.From Nulcear Garden:
The water slowed the subatomic particles or neutrons that were cast out from the uranium so that they could hit-and split-other atoms. Without the water, the neutrons would move so fast that they would just bounce off, like skipping a rock across the water, and not produce nuclear chain reactions. When the heat from the reactions became too great, the water turned to steam and stopped slowing the neutrons. The reactions then slowed until the water cooled. Then the process could begin again.
Scientists think these natural reactors could have functioned intermittently for a million years or more. Natural chain reactions stopped when the uranium isotopes became too sparse to keep the reactions going.
When more than a 'critical mass' of uranium containing the fissionable isotope is gathered together in one place there is a self-sustaining chain reaction. The fission of uranium atoms sets free neutrons that cause the fission of more uranium atoms and more neutrons and so on. Provided that the number of neutrons produced balances those that escape, or are absorbed by other atoms, the reactor continues. This kind of reactor is not explosive; indeed it is self-regulating. The presence of water, through its ability to slow and reflect neutrons, is an essential feature of the reactor. When power output increases, water boils away and the nuclear reaction slows down. A nuclear fission reaction is a perverse kind of fire; it burns better when well watered. The Oklo reactors ran gently at the kilowatt power level for millions of years and used up a fair amount of the natural Uranium-235 doing so.Both of these sites point that that:
Once the natural reactors burned themselves out, the highly radioactive waste they generated was held in place deep under Oklo by the granite, sandstone, and clays surrounding the reactors’ areas. Plutonium has moved less than 10 feet from where it was formed almost two billion years ago.8Beyond the sheer fantasticness of such a natural occurrence, the Oklo nuclear reactors demonstrate that it is possible to safely store nuclear waste products for significant periods of times.
Today, manmade reactors also create radioactive elements and by-products. Scientists involved in the disposal of nuclear waste are very interested in Oklo because long-lived wastes created there remain close to their place of origin. ...
The radioactive remains of natural nuclear fission chain reactions that happened 1.7 billion years ago in Gabon, West Africa, never moved far beyond their place of origin. They remain contained in the sedimentary rocks that kept them from being dissolved or spread by groundwater. Scientists are studying Yucca Mountain to see if the geology there might play a similar role in containing high-level nuclear waste.