Two billion years ago, planet Earth had natural nuclear fission reactors burning inside its crust. You are looking at the remains of one of them, located in Oklo, Gabon.
These reactors are thought to have happened by natural causes as a result of the abundance and density of Uranium-235 - a natural fissile uranium isotope present in our planet and throughout the solar system. Two billion years ago, there were 3000 atoms of U-235 per 100,000 atoms of other materials.
In addition to having at least 1 per cent of U-235 and being big enough to sustain the reaction, these natural nuclear furnaces were required to have a low concentration of neutron absorbers (like cadmium) and a high concentration of a moderator material (like water or carbon). Nowadays, there are no natural reactors happening on Earth, because there are not enough quantities of this material.