Uranium ore reaches a lab with fewer fuel atoms than expected. Follow the trail back to Oklo in Gabon and you find a reactor nobody built. The rock once split atoms by itself.
Splitting a heavy atom is fission. It releases energy and small particles that can split more atoms. Long ago, the ore held enough of the useful kind of uranium to keep that chain going.
Groundwater slowed the particles so they could strike the next atoms. Then the rock got hot and boiled the water away. The reaction stopped. As it cooled, water seeped back, and the chain could start again.
Think of a geyser running on uranium. Gas trapped in minerals suggests bursts of about 30 minutes, followed by roughly 2.5 hours cooling down. The rock carried its own stop-and-start switch.
Scientists place the reactors between about 1.7 and 2 billion years ago. The exact age is still debated; the Clock uses a point within that range. You can study what remains in the ore today, including waste from the reactions. Much stayed close to where it formed, though some substances moved more than others. The missing fuel atoms were spent inside wet rock.
