This photo shows part of a natural nuclear reactor-- an underground uranium deposit where a chain reaction occurred spontaneously. In fact, such a natural reactor was predicted, beginning in 1956, and then discovered in 1972.
To find the reactor, physicists search for a deposit of uranium with a slightly lower concentration of U-235 than is found elsewhere on Earth. This U-235 deficit would be caused by the chain reaction of a small fraction of the U-235 nuclei. To learn more, visit The Oklo Fossil Fission Reactors and also this Astronomy Picture of the Day page.