The remains of 17 natural nuclear reactors have been discovered in a uranium mine in Oklo, Gabon. Nuclear waste of the same type and percentage as modern nuclear reactors is clearly present: the dreaded plutonium-239, which in its decay process generates a whole series of fission byproducts found exactly (within 1 ppm) in the percentages predicted by theory. Considering that the reactors are 1.7 billion years old, it can be concluded that the laws of physics (at least the weak interaction) have changed by less than 1 ppm in the last 1.7 billion years!
