In 1972, some French scientists visited the Oklo mine in Gabon, Africa to test its Uranium ore. Although there are different Uranium isotopes found in nature, an Uranium ore should contain the same ration of these isotopes no matter where this ore was found. But this particular ore from the Oklo mine had significantly less Uranium 235. This isotope is the one used to produce energy, sustain a chain reaction, make bombs... So instead of 0.72%, they found 0.717%... Yeah, this slight difference makes a big deal with Uranium. As a matter of fact, the amount of the missing Uranium was enough to make 6 A-bombs.
Since no one could have "stolen" this Uranium 235 isotope, scientists came up with a different story to explain where is the missing staff... A natural and fully-regulated chain reaction! Which is double awesome!
It seems that somehow a speeding neutron found its target, the core of an Uranium-235 isotope. The core of the Uranium atom splits into some other sub products, including a couple of more neutrons which found other Uranium atoms and the reactions went over and over again. But an uncontrollable reaction means that the neutrons move way too fast and cannot find their next Uranium target. And this would have stopped the reaction.
But the mine had also cold ground running water flowing. The water would then act as a slowing agent, effectively slowing down the speed of the neutrons, giving them time to hit other atoms of U-235 and sustain a reaction. This process of course released huge amounts of energy heating up the water to high temperatures. And before the chain reaction went out of control blowing the mine into pieces, the water turned so hot that it rapidly turned into a steam. Without the liquid water the neutrons could speed up, missing again their U-235 target and the chain reaction would slow down.
This, guys, is nature's best PID! Check out this video.