Oklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African country of Gabon. Several natural nuclear fission reactors were discovered in the uranium mines in the region in 1972.
I assumed the natural reactors might still be fissioning, but the Wikipedia page says this occurred about 1.7B years ago. They know they were reactors because the cite has a lower concentration of uranium-235 than expected and also reaction by-products.
Oklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African country of Gabon. Several natural nuclear fission reactors were discovered in the uranium mines in the region in 1972.
https://en.wikipedia.org/wiki/Oklo