When you think about nuclear fission reactor, you probably thought of man-made nuclear energy power plants (the Springfield Nuclear Power Plant from The Simpsons, perhaps).
But did you know that Mother Nature actually had set off nukes on Earth about 1.7 billion years ago?
Ethan Siegel of Stars with a Bang wrote about Earth's First Nuclear Reactor:
The Oklo fission reactors are the only known examples of a natural nuclear reactor here on Earth ... For approximately 30 minutes, the reactor would go critical, with fission proceeding until the water boils away. Over the next ~150 minutes, there would be a cooldown period, after which water would flood the mineral ore again and fission would restart.
This three hour cycle would repeat itself for hundreds of thousands of years, until the ever-decreasing amount of U-235 reached a low-enough level, below that ~3% amount, that a chain reaction could no longer be sustained.
Read more about the Oklo Mine natural nuclear fission reactor over at Wikipedia
Photo: US Department of Energy
Like, what was is built for? It wouldn't have been an intense power source. Wikipedia says "less than 100 kW", so enough for about 15 US homes. We know how to make more powerful reactors, or make a small solar farm for that amount of energy.
Scientists have done many tests to understand the chemical changes, including looking at the distribution of different element types. If there were any supporting infrastructure, like cooling pipes or power transmission lines, they should have left a signature, eg, increased copper concentration.