In 1972 a routine check at a French plant found uranium from the Oklo mine in Gabon slightly short of its fissile kind: about 0.717 percent, where natural uranium gives 0.720 almost everywhere. The missing part had been used. About two billion years ago, when that kind of uranium was far more common, groundwater seeped into the seam and slowed its neutrons enough to start a chain reaction, with nobody there to start it. Xenon trapped in the rock suggests how it ran, though the details are still argued: about half an hour on, until the heat drove off the water, then two and a half hours off, until the water came back. It seems to have kept that up, on and off, for something like 150,000 years, with no controls except the thing it needed leaving whenever it got too hot. Then it stopped, and stayed stopped for two billion years, until somebody looked at the third decimal place and did not let it go.