In a mine in Oklo, near Franceville in southeastern Gabon, scientists in the 70s found a rich vein of uranium ore. However, when they tested it, they found it was weirdly missing a certain isotope – it had a makeup more like spent nuclear fuel than natural radioactive material. This was something that shouldn’t happen with untouched natural uranium.
It turns out that two billion years ago, specific conditions underground created a natural nuclear reactor, one that used water to transfer heat and moderate output much the same way that power plants do. That used up the radioactive isotopes, so by the time humans evolved, figured out nuclear power, and went looking for uranium, they were long gone. Here’s a good explanation of how it worked: