by: Eben Harrell
George Frey / Bloomberg News / Getty Images
Some 2 billion years ago, a natural-uranium deposit deep underground in what is now the west-central African country of Gabon spontaneously went critical. In the only known case of a nuclear-fission reactor forming naturally on earth, the Gabon deposit fissioned just like a modern-day power plant. As well as generating a substantial amount of heat, the uranium also produced a huge quantity of radioactive waste products, including around four tons of plutonium. Because this naturally occurring nuclear waste was buried deep underground, it remained remarkably well confined as it decayed over the course of millennia.
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