Two billion years ago, where the Gabon Republic is now located in Africa, there existed a huge nuclear reactor that was operative for five hundred thousand years.
Oklo is the site of a uranium mine in the Gabon Republic. France obtains much of the uranium used in its nuclear program from this mine. In 1972 when uranium ore from this mine was introduced into a French gaseous diffusion plant, it was discovered that the feed uranium was depleted below the 0.711 w% of ordinary natural uranium. It seemed as if the uranium had already been used. The French government announced the finding and took the whole world by surprise.
Scientists investigated the uranium mine and the results were made public at a conference of the International Atomic Energy Agency. The uranium indeed had been used. One may wonder what exactly happened at Oklo?
French scientists found traces of fission products and fuel wastes at various locations within the mine area. These observations were puzzling at first because it is not possible for natural uranium to go critical, except under very special circumstances such as the presence of graphite or heavy water as a moderator, neither of which could reasonably be expected to have been present in the vicinity of Oklo.
The half-life of U235 is 7.13E8 years, which is considerably shorter than the half-life of U238 at 4.51E9 years. Since the original formation of the earth, more of the U235 had decayed than the U238. This means that the natural uranium ore had a much higher uranium concentration many years ago than it does today. Indeed, it is easy to show that about 3 billion years ago the U235 concentration was in the neighborhood of 3 w%, sufficiently high to reach the critical point for a chain reaction to occur in ordinary water, which was present near Oklo at that time.
Surprisingly, this uranium mine's nuclear reactor was well designed. Studies indicate that this reactor was several miles in length. However, for such a huge nuclear reactor, the thermal impact to its environment was limited to 40 meters on all sides. Even more astonishing is the fact that the radioactive wastes have still not migrated outside the mine site. They are held in place by the surrounding geology.
Faced with these findings, scientists consider the mine to be a 'naturally occurring' nuclear reactor. The Oklo reactor has been documented for its importance as an analogue (a structural derivative of a parent compound) in the disposal of nuclear fuel wastes. But few people are bold enough to go one step further.
As a matter of fact, many people today know that the reactor is a relic from a prehistoric civilization. It's probable that two billion years ago there was a fairly advanced civilization living at a place now called Oklo. This civilization was technologically superior to today's civilization. Compared to this huge 'natural' nuclear reactor, our current nuclear reactors are far less impressive.