The remnants of nuclear reactors nearly two billion years old were found in the 1970s in Africa – these reactors are thought to have occurred naturally. When enough uranium is brought together into one place, fusion reactions occur, generating vast amounts of heat. No natural reactors exist today, as the relative density of fissile uranium has now decayed below that needed for a sustainable reaction. This reactor is in Gabon – deep underground, the traces of yellow uranium oxide are clearly seen. By-products are being used today to probe the stability of the fundamental constants over cosmological or very long timescales and to develop more effective means for disposing of human-manufactured nuclear waste.