Twice this week I've encountered people who discuss riding down a "zipline" in Costa Rica. One of my friends from work will go later this year. I had never heard of this. When she first mentioned it I thought it meant they insert a hook in the seat of your pants and dangle you over a ravine. Actually, you get strapped in a harness and hang from a cable strung horizontally through a rain forest. The cable gently slopes down, so you ride in this harness down through the forest.
How fast do you go? Twenty miles per hour? Fifty? One hundred? Do you hit any birds? How far does it go? How long does it take? Do you feel like you have to go to the bathroom halfway through the ride? What are your options at that point? Can you stop sliding? What if somebody comes along behind you at 100 mph and you're stopped, admiring the view?
Imagine you're not on the zipline but hiking through the rain forest and overhead, every five minutes, you hear the steady shriek of somebody whooshing down a zipline at high speed. The Doppler effect lends a high pitch to their scream as they approach, and after they pass, screaming loudest, the pitch of their scream lowers.
How do you stop? Is the lower end of the cable tied to a big tree, with bark worn away from the impact of so many high-velocity tourists? Is the tree leaning away from the cable after so many collisions? Maybe the cable's end is suspended so you just go flying off and land in a pile of fellow travelers. Maybe the cable gradually approaches the ground so you have to start pedaling your legs before you land, like a Fred-Flintstone-powered plane.
This is why I never travel anywhere. One minute you're in Italy touring St. Peter's Basilica and the next minute you're giving in to peer pressure and riding a zipline through a grotto full of dead popes.