I'd always wondered about this idea, which I recently ran across: a natural nuclear reactor.
Of course, if you understand the principle for how uranium fission works, you know that if enough uranium gets together in one spot, you get what's called 'critical mass' and you will get a fission reaction. So there's no reason, in principle, why it couldn't happen entirely by accident, in the natural world – some uranium deposit getting too dense, by natural geological forces, and, bingo, fission! I just assumed, based on my quite limited understanding, that the odds were too low for it ever actually happen. Yet apparently, it has, and it has been scientifically confirmed. I found some links that led me to this article, at an architecture-related blog I read sometimes. It all seems quite remarkable.
[daily log: walking, 7km]