OMG I want to tell you about the coolest thing I've learned about in a hot minute! (Maybe everyone already knows about this, but it's new to me & I LIKE LEARNING STUFF LET'S GO ON A JOURNEY OF EXPLOSIVE DISCOVERY TOGETHER)
SO. Under certain precise conditions, natural deposits of uranium can develop spontaneous nuclear fission chain reactions identical to the kind we make on purpose in modern nuclear reactors
*This has already happened at least once.* About 1.7 billion years ago in Oklo, Gabon