In May 1972 in a uranium enrichment plant in France, scientists examining ore from a mine in Gabon, West Africa, discovered that a natural nuclear reactor had spontaneously manifested in that region in the Earth’s primordial past, churning out approximately 100 Kw worth of energy continuously for a few hundred thousand years about 1.7 billion years ago.

There are two theories about how the Gabon reactor worked, although both assume a cycle of chain reaction, cessation, cooling, repeat, over a period of thousands of years, until the fissionable material was exhausted.

One theory proposes that the uranium was covered with groundwater, which moderated the neutrons and provided an environment that supported a chain reaction. The energy generated eventually heated the groundwater to boiling, and it steamed away.

Photo via Curtin University.
Diagram via Scientific American.

Quoted text via Gizmodo.