Roughly 1.7 billion years ago, though a crack in the Earth, a seep of groundwater began to boil. No volcanoes were near, no lava. No, under what is modern day Gabon, a rush of water had begun to act as a neutron moderator for a vein of uranium rich minerals. The heat generated by nuclear fission caused the groundwater to boil away, which slowed and stopped the reaction. 3 hours later, the groundwater would rush in again, and the cycle repeated. Over, and over, the Earth's own and only naturally occurring nuclear reactor ran naturally for hundreds of thousands of years, until eventually it ran out of enough fissile materials to sustain a chain reaction.