Scientists are hoping to create tiny black holes here on earth, which they are pretty sure will evaporate almost immediately due to Hawking radiation. While they consider the risk to be almost nonexistent, I can envision some scientist saying "cool -- wonder if we can make a little bigger one". My question is how big would one have to be before it was a danger to the planet, and is it theoretically possible to make such a one in a lab? If so, I can imagine every advanced civilization eventually trying this experiment and ultimately destroying themselves. If that is the case it could explain why we have not heard from any advanced civilizations.