The Oklo-reactor in Gabon, Africa is one of the most intriguing geological formations found on planet Earth. Here, naturally occurring fissile materials in two billion year-old rocks have sustained a slow nuclear fission reaction like that found in a modern nuclear reactor.
Image by Wikipedia/ ShinRyu Forgers, CC BY-SA 4.0SHINRYU FORGERS
Uranium-235 is a radioactive element with a half-life of 700 million years. Traces of it are found in almost all rocks, especially magmatic rocks, and its decay is believed to be one of the sources of Earth's inner heat. Because it decays over time at a constant rate, its concentration in the Earth's crust is almost everywhere the same - except in Oklo.
The Oklo-Formation, a succession of sandstone and siltstone, was deposited two billion years ago by a large river. Microbial activity of the first lifeforms caused the element uranium, derived from weathered magmatic rocks, to become concentrated in certain layers of the sediments. Later tectonic movements buried the layers deep underground.
Full story available at forbes.com.