If you haven't seen downhill biking, this is a good introduction to it.
A guy who won 1st place at the Snow Summit Downhill race in Big Bear Lake, CA, strapped a GoPro to his helmet and takes along for the ride. The race is five minutes long, and it's a testament to how much skill these guys have on their bikes.
Combine speed, sharp turns, jumps and a bunch of close calls with tree trunks and you got yourself one hell of a ride, even if you are just watching it on a video like this.