To expand on an old cliche:
Why do you climb that mountain?
Because it's there.
Why do you climb that vertical treadmill?
Because the mountain isn't there.
You've seen those knob-covered climbing walls at gyms and arcades. They may be fun, but when you get to the top, there's no place to go but down. Going down is a lot scarier than going up, because you have to look down. The Treadwall is a climbing wall that never ends. You don't have to stop climbing, you don't have to climb down, and you can't fall far, since you're never more than a couple of feet off the ground. And get this- it doesn't even have a motor. The weight of the climber pulls the wall down! You stop and start anytime you're ready, and set the pace by your climbing. The resistance can be loosened if you want to climb really fast, and the slope is adjustable. See a video here.

The Treadwall comes in four foot and six foot sizes and can be upgraded to a weatherized version for outdoor use. It costs around $10,000, so don't expect to see them in backyards anytime soon, but that price is feasible for businesses such as gyms and arcades.