Ancient nuclear reactors?

It sounds incredible, but apparently it's true - in the 1970s, scientists discovered evidence of two billion year old nuclear reactors in Africa.

These reactors weren't built by aliens or now-lost Atlanteans, of course - they occurred naturally. It turns out that billions of years ago, uranium was present in the earth's crust in sufficient quantities to spontaneously undergo fission, given certain other prerequisites. From Scientific American:

Paul K. Kuroda, a chemist from the University of Arkansas, calculated what it would take for a uraniumore body spontaneously to undergo selfsustained fission. Amazingly, the actual conditions that prevailed two billion years ago in what researchers eventually determined to be 16 separate areas within the Oklo and adjacent Okelobondo uranium mines were very close to what Kuroda outlined. These zones were all identified decades ago.

You can read the full article here. It concludes, interestingly, that there may have been yet other naturally-occurring nuclear reactors in our planet's past. Go figure.