AT A GLANCE
Why go?
In the dense forests of southeastern Gabon lies the only place on Earth where nature built a nuclear reactor — roughly 1.8 billion years before humans split the atom for the first time. The Oklo Natural Nuclear Reactor is not a single machine but a series of fossilised reaction zones buried within the uranium-rich sandstone of the Franceville Basin, where groundwater, high-grade ore, and a younger planet's more abundant fissile isotope combined to ignite a self-sustaining chain reaction that pulsed for hundreds of thousands of years.↗↗
Discovered in 1972 when French scientists at a uranium enrichment plant noticed impossibly low concentrations of uranium-235 in ore samples from the Oklo mine, the site has become one of the most celebrated geological phenomena in the world — a natural proof that nuclear fission preceded humankind by eons, and a living laboratory for understanding how radioactive waste remains locked in rock for billions of years.↗↗
A Two-Billion-Year-Old Chain Reaction
Around 1.7 to 1.8 billion years ago, during the Paleoproterozoic Era, uranium deposits in what is now southeastern Gabon reached a concentration of fissile U-235 of roughly 3% — about the same enrichment used in today's commercial nuclear power plants. When groundwater flooded the ore bodies, it served as a natural neutron moderator, slowing neutrons enough to sustain a fission chain reaction. The reactors operated in cycles: heat would build until the water boiled away, shutting down the reaction; cooling would then let groundwater seep back in, restarting it. Xenon isotope analysis shows this stop-and-go rhythm repeated on a cycle of approximately 2.5 hours — sustained continuously for hundreds of thousands of years.