"Ah, well. I'm sure they have perfectly logical explanations for everything." Another smirk. Of course they would, if only to convince people not to keep trying to find the truth. "What happened? Now that's an excellent question. I haven't looked much into it, but there didn't seem to be much to look into. You'd think if something nearly wiped your planet to extinction you'd make good records of it in the hopes of preventing a repeat of the incident, wouldn't you?" The Doctor shook his head in disbelief. "Unless it was decided that whatever caused it should be forgotten. If people found something... and then removed all mention of its existance, then it would be difficult to find it again, wouldn't it?"