[en] The discovery in summer 1972 by a French scientist that the ²³⁵U-concentration of uranium from the Oklo Mine in Gabun/Africa was slightly less than what one would expect from natural uranium led to a very remarkable discovery: the Oklo phenomenon. At least six places have been located up to now at the Oklo Mine at which nuclear chain reactions took place about two billion years ago. The nuclear-physical data of the Oklo phenomenon are largely comparable with those of our present modern light-water reactors. - Several elements found in the uranium lenses with their high uranium content show isotopic compositions quite different from those which we know for naturally occurring elements. The deviation in isotopic composition of, e.g. neodymium, ruthenium, europium etc. enables the modern chemist and physicist to get a very clear and reliable picture upon the properties of the ancient nuclear reactor (e.g., neutron flux, time and duration of the event, burn-up, production of plutonium via neutron capture of uranium). - This article will give a comprehensive description on what we presently know of this exceptional event. A more detailed discussion is devoted to the nuclear-physical properties of the Oklo phenomenon and the behaviour of the fission products part of which - and this is very astonishing - have not migrated even a few millimeters during the past nearly two billion years. (orig.)