Favorite Mind Boggling Calculations?

One of my favorite things about science are the mind boggling calculations and equations that come along as a by-product of complex theories. I mean just check out Einstein's Field Equations or Graham's Number (which is more theoretical maths, but anyway, I digress). So I'd like to know what are some people's favorite complex calculations, or just extremely trippy theories. This one is definitely more for the physics/maths oriented people, but I'm sure there are some crazy biological equations that I haven't seen before.