"Don't look down," I tell myself firmly. "Whatever you do, don't look down."
Which, of course, is the first thing I do, and I'm paralyzed by fear.
Suspended above a plunging chasm in a disused mine deep below a Welsh mountain, my stomach is lurching between vertigo and claustrophobia.
When I first heard about this place, playfully named Bounce Below, I had in mind something considerably tamer.
Bounce Below is the latest addition to the Llechwedd Slate Caverns, a 176-year-old stone quarrying site near Blaenau Ffestiniog, a town in the rain-lashed hills of north Wales.
The caverns have long attracted visitors with tours of Victorian mine shafts and subterranean rides down what's advertised as the steepest cable railway in Britain.
It was all quite serious, until someone decided it might be a fun idea to string up some elasticated nets.
Monster spider web
Envisioning some kind of underground soft play center, I board the Llechwedd mining train and begin my descent.
Soon I find myself in a cavern twice the size of London's St. Paul's Cathedral.
Nets stretch across three levels like a web spun by a monster spider.
Multicolored LEDs cast eerie bouncing shadows on the rocky walls as berserk grownups in prison inmate outfits bounce, hop and tumble along flimsy net chutes, playing tag with staff in gleaming fluorescent jackets.
The highest trampoline is suspended 180 feet (55 meters) above the cavern floor.
I make my way onto a mid-level net.
A member of the friendly staff leads me by hand to the center.
That's when I look down.
Totally wired: Zip World Velocity
Courtesy Bounce Below/Zipworld
The terror is paralyzing but there's no time to think about it.
You fall, you pick yourself up -- and fall down again.
People race around, attempt gymnastics, lie down, hurtle down the slides.
At the top of the slides, the drill is to sit on the edge, cross arms, then let go.
So that's what I do.
It's over in a few seconds, and, I'm relieved to find, it's thrilling.
Almost a convert, I'm ready for the big one -- the slide down from the middle net to the lowest level.
I've just about mustered up enough courage when a hyperventilating bouncer climbs back up the slippery narrow chute complaining of claustrophobia.
My reserve of enthusiasm evaporates.
Eventually, my allotted hour ends and I board the train back to the surface.
I can't say I'm unhappy to be out, but I'm happy to have tried this incredible, unique experience.
More thrills at surface level
Back on the surface, there's more fun to be had.
The carved-out slate quarries and mountainsides of Llechwedd are also home to Zip World Titan -- Europe's first multiple zip line, it allows four people to race each other at crazy speeds.
The next day I'm wearing baggy overalls, being strapped into a harness and peppered with safety instructions.
"Put your arms out if you want to slow down," I'm told. "If you start spinning midair, pull at the straps. If you come to an abrupt halt midair, don't worry, someone will eventually come and push you along."
Courtesy Bounce Below/Zipworld
The gates open and we're zipping off, racing each other.
The ride is exhilarating, the panoramic view of the Welsh mountains distracts me from the thought that I'm dangling 1,500 feet above the ground.
It takes less than a minute and already I want it to go higher, faster.
I try a second line, this one going over a drop of 2,450 feet at speeds of up to 70 mph (114 kph).
We fly over the quarry, past the mountains and straight into the arms of the waiting instructors.
A short trek later and I'm flying again at great speed over the tiny houses back to where we started.
The discovery of my new aerodynamic self makes me burst with pride.
Now where did I leave my cape?
Bounce Below and Zip World Titan, Llechwedd Slate Caverns, Blaenau Ffestiniog; Bounce Below (£20/$32); Zip World Titan (£50/$80); +44 1248 601 444 There's a second zip line experience nearby, said to be the longest in Europe. Zip World Velocity, Penrhyn Quarry, Bethesda; £60 ($95); +44 1248 601 444