Watch Us Glow

Standing inside a natural nuclear reactor (long depleted, fortunately):

http://apod.nasa.gov/apod/ap021016.html

Back in the Seventies, researchers trying to explain why there was so little U-235 in several sections of ore realized that 1.7 billion years ago - when the natural concentration of U-235 was higher - ground water moderated the naturally rich ore and created a nuclear reactor. More info here:

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor