Today nuclear reactors generally work by using uranium artificially enriched in the one isotope, U-235, that fissions the most so that an energy-producing chain reaction can take place. Without enrichment, you can pile up tons of uranium and it won't make any perceptible heat. Nevertheless, in 1972 the remains of a natural, spontaneously formed uranium reactor were found in ancient rocks of the African nation of Gabon, in the Oklo uranium mine.
What made such a thing possible was that in the distant past uranium was naturally enriched in U-235, that is, less of it had decayed away by nuclear fission. About 1.7 billion years ago, to be more precise, a natural deposit of uranium ore was radioactive enough to generate about 100 kilowatts of heat, off and on, for more than a million years.
Geologic forces gathered the uranium together. First a layer of sandstone was infiltrated by uranium-bearing groundwater, leaving a relatively thin sheet of uranium-oxide ore. Then the rocks were tilted, and as they eroded downward the groundwater concentrated the uranium minerals, sweeping them downward within the sandstone until a thick stripe of ore was built up. That's when things heated up.
To understand what happened next, you need to know a little about nuclear reactors. The nuclei of uranium atoms normally decay with the release of energetic neutrons—so energetic that they fly away without interacting with other uranium nuclei. The neutrons need to be slowed down before they can start splitting other uranium nuclei, which release more neutrons and start a feedback cycle. Something needs to moderate the neutrons. The first artificial reactor, built in 1942, used balls of enriched uranium spread out inside a large pile of graphite blocks, which served as a moderator.
But water acts as a moderator, too. At Oklo there was a lot of water, probably a river flowing above the buried orebody. The water allowed the nuclear interactions to reach the critical point, and the reactor began to work. But as it heated up, the water turned to steam and flowed away. With the moderator gone, the chain reaction stopped and did not start again until the orebody cooled and the water returned. This simple feedback cycle kept the Oklo reactors (there were at least a dozen of them) active until the U-235 was depleted. That took about a million years. When the Oklo mine was producing ore in the 1970s it was that telltale depletion of U-235, unheard-of in nature, that tipped scientists off.
A remarkable thing about the Oklo reactors is that the highly radioactive waste products stayed put without the elaborate containment we use today on nuclear power plant waste. More than a billion years later, everything is contained within a few meters of its source.
Recently a team of scientists took advantage of this excellent preservation and studied the isotopes of xenon gas—a product of uranium decay—trapped in phosphate minerals at Oklo. Led by Alex Meshik of Washington University of St. Louis, they reported in 2004 that the reactor went through eight cycles a day, running for 30 minutes then shutting down for two and a half hours. The whole thing is reminiscent of geysers.
Why was uranium so much more radioactive then? That is a deep question that points to the very origin of the solar system. The formation of the planets (and the Sun) from an original cloud of dust and gas apparently was triggered by the explosion of a nearby supernova. Only a supernova can manufacture elements heavier than iron, including uranium. With a half-life of 700 million years, U-235 started out making up nearly half of all uranium when the solar system began some 4560 million years ago. Many shorter-lived radioisotopes that existed in the beginning, like aluminum-26, have become extinct. We know of their former existence by the presence of their decay products in ancient meteorites—nuclear fossils.