Natural Reactor
Nuclear power plants were created with a lot of thought and technology, but the fundamental concept is not one that depends on technology. In fact, there are 17 naturally occurring nuclear fission reactors in Gabon, which is in Western Africa. They spontaneously developed two billion years ago and generated 1,000 light bulb-worth of electricity.
For upwards of a million years, these natural reactors produced electricity, and radioactive waste was carefully stored at the locations as well. This proved that geological storage could occur safely and securely, which has important ramifications for us in the current day. Almost equivalent to artificial reactors, natural reactors generate nuclear processes. The energy released when uranium atoms split apart was slowed down by the presence of water, sustaining the process for a long time.