The ideal combination of circumstances required for such an event—a criticality—has even aligned spontaneously in nature: in ancient subterranean deposits of uranium found in the African nation of Gabon, where groundwater acted as a moderator. There, self-sustaining chain reactions began underground two billion years ago, producing modest quantities of heat energy—an average of around 100 kilowatts, or enough to light a thousand lightbulbs—and continued intermittently for as long as a million years, until the available water was finally boiled away by the heat of fission.
