There's a black hole at the center of the Earth.
It's an interesting idea, and the more I read about black holes in general, the more sense it seems to make that small black holes could be an explanation for everything from planets forming out of dust clouds to the rings of Saturn.
This link is an excellent starting point. She's pretty persuasive about the pervasiveness of black holes in our night sky as well as under our feet. It's kinda weird. :) Go read.