Finally I reached the top, pale-faced and panic-eyed. As I wiggled my toes over the edge, muttering 'Don't look down', and leaned into the void to grab hold of the surprisingly heavy bar, my mind went blank. I must have jumped, because I was suddenly aware of a feeling of complete freedom and unadulterated joy as I flew across the vast room. I'm no adrenaline junkie, but it was a rush like nothing I've experienced before.