Also fun fact. For nuclear power the percentage of U-235 needs to be 3-5 percent. It is currently about 0.72 percent in natural uranium.
About 1.7 Billion years ago the natural percentage WAS around 3-5 percent. In certain places in the world there were areas of natural uranium that got flooded with water, which is a neutron moderator. Allowing the natural neutrons to slow down and collide with other uranium atoms, causing a chain reaction. Creating [Natural Nuclear Reactors](https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor). We found them because as we were mining there was missing U-235, it was coming out at .6% instead of .72%. Which was a major issues as the first though is where did it go? Did someone steal it and fudge the percentages.