Wouldn't mind a glowing cat. Stops me from tripping over her when I have to run to the toilet in the dead of night..

In the past. Earth no longer has enough of the applicable isotope for them to happen. But it's cool we found evidence at least one existed! https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

There's a guy on Reddit in the space subreddits that is sure there is a natural reactor on Mars now, based on a paper by a kooky scientist.