The earth formed a nuclear reactor by chance. Two billion years ago. Really.
Most programs about nuclear reactors stress what a difficult and intricate process it is to make one. The right elements have to be obtained and enriched. The whole thing has to be concentrated. And to top it off, the entire system has to be kept in balance so that it neither shuts down nor blows up. It's tough to do. But it's been done without so much as an engineer to build it.
Africa is now not only the birthplace of humanity, but also the birthplace of terrestrial nuclear power, the eventual doom of humanity. In Gabon, in the bed of a long-gone river, uranium sites were discovered. Uranium deposits are always of interest because of the potential of Uranium 235, which can be used to produce nuclear power through fission. This deposit was strangely depleted, though. After puzzling over what could have done it, scientists realized that something had already been using the Uranium 235 to produce power.
This is where most rational people would have decided that clearly aliens had built a nuclear power generator there to fuel the flying cars which formed the pyramids, but scientists came up with something even more unbelievable.
Over two billion years ago, a river ran through Gabon towards the sea. Around this river were scattered deposits of Uranium. Although the river eroded away rock, forming a sandy riverbed, it did not have a lot of luck tearing away at the heavy Uranium deposits. What turned this around was the evolution of blue-green algae. The algae pumped out oxygen. This formed uranium oxides, which dissolve in water. This dissolved the Uranium deposits around the river, and allowed uranium rich water to wash over the sand at the bottom of the river. Reaction with the sand tore apart the uranium oxides, dumping the uranium into the sandy, but hardening, river-bottom sludge.
Geological forces tilted the sandstone at the bottom of the river, forming a crease along the river bottom. The still-rushing water washed the uranium laden sandstone downward, concentrating the uranium even more. The resulting massive deposit of Uranium was primed for fission.
For fission to occur, a neutron has to fly into the nucleus of the Uranium 235 atom and attach itself there. This makes the atom very unstable, and it soon splits, in the process releasing a lot of energy and at least two more neutrons. These go find themselves other Uranium 235 atoms to bother, and the reaction is sustained.
A few conditions have to be met for this to happen. The Uranium 235 atoms need to be close to each other, and present in great quantities, but not great enough to make the reaction go so fast that they blow the whole thing to smithereens. (It can happen, but it would be more of a bomb than a reactor.) Usually this happens by increasing the quantity of Uranium 235 in any given sample of uranium. In Oklo, the site of the ancient reactor, the sediment held such a great quantity of Uranium 235 that it could sustain a reaction.
Another condition has to be met, as well, for fission of uranium to occur. When neutrons fly out of a splitting nucleus, they are traveling at incredibly high speeds. These speeds are often too high for them to bury themselves in the nuclei of other atoms. Something needs to slow them to the right speed. In Oklo, that something was the river itself. It slowed the neutrons until they were able to cause the reaction. It also regulated the reaction enough that it was going for hundreds of thousands of years. Whenever too many uranium atoms split, they released enough energy to boil off the water in the river. This caused the neutrons to fly off willy-nilly, without causing any more reactions, and the whole thing shut down until more water came in.
The Oklo reactors kept going, off and on, for about a million years. They released a massive amount of power, in a relatively contained way, by, and this bears mentioning once again, complete chance. It almost seems unfair to modern engineers.