Cluster Ballooning is air travel achieved by means of tying numerous helium balloons to yourself. I knew about Larry Walters' famous 1982 cluster balloon flight in which he took off from the LA area on a lawn chair tied to helium balloons, so I knew it was possible to do. But I didn't think that people did this regularly as a sport. Apparently they do. It actually looks like fun (the site has some great pictures).