- A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred.
- Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of thermal power during that time.
There's now evidence of historical natural nuclear fission reactors on Mars.
Lets not forget the natural fusion reactor: Also interesting is that one of the early arguments for young earth was that it should have cooled down in the time since a billion years ago. It turns out that the decomposition of uranium and such contributes to keeping the earth warm/molten.
my favorite part: Published in 2010, a controversial hypothesis[note 1] about the origin of the Moon proposes that the Moon may have been formed from the explosion of a georeactor located along the core-mantle boundary at the equatorial plane of the then-rapidly rotating Earth, 4.5 billion years ago.[10] While the "explosion of a georeactor hypothesis" would explain in part the compositional similarities between the Moon and the Earth, the commonly accepted hypothesis remains the giant impact hypothesis which explains these compositional similarities as well if not better.Origin of the Moon hypothesis