Natural Nuclear Reactor

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Nuclear fission reactors are not exactly a human invention. Under the right conditions naturally-occurring fissile ore can spontaneously begin a nuclear chain reaction. You can read more about that here:
Natural nuclear fission reactor

The Wikipedia article specifies that it refers to natural nuclear fission reactors, presumably to contrast the concept with the large number of natural nuclear fusion reactors (i.e. stars) in the universe. You could just as easily refer to the fission reactors as such and the stars as thermonuclear reactors (since fission can happen at any temperature while fusion requires obscenely hot places to work), so I left out the "fission" part out of this post's title. Whatever they're called, the basic idea is that whenever a sufficient amount of rich uranium ore gathers in a place with minerals that can moderate the speed of the reaction-sustaining neutrons without absorbing them completely, a self-sustaining nuclear reaction will begin. That's just how physics seems to work in this universe.

While stellar thermonuclear reactors abound in the cosmos, only one natural nuclear reactor is known to have existed on Earth. Nearly two billion years ago, in what's now west Africa, all the conditions were right for the Oklo reactor to begin reacting. This Scientific American article provides a good overview of the fascinating details of Oklo's discovery and history. It looks like the reactor complex operated in an on-off cycle for a few hundred thousand years, running while groundwater seeped into the sandstone encasing the uranium ore, until the temperature rose enough to boil the neutron-moderating water away. Eventually fission products poisonous to further reactions built up until the reactor shut down for good 1.7 billion years ago. Surely this wasn't the only place this happened, but only at Oklo did the scene remain undisturbed until uranium miners came along in the mid-20th Century.

The numbers at Oklo give some insight into the tremendous energy that heavy atoms pack. Since the reactor complex wasn't designed for power, geologists and nuclear physicists estimate that it averaged about 200 kilowatts during its life, or about 270 horsepower in the deeply awkward and inconsistent units of the Imperial system. That might not sound like much, but consider that during Oklo's active life it consumed about five tons of uranium, converting it into lighter fission products like neodymium and ruthenium. Five tons of nuclear fuel sustained a power equivalent to a reasonable sports car engine at full throttle for hundreds of thousands of years. Oil might be the backbone of our energy economy for now, but it ain't got nothin' on uranium.