They call it the Death Slide, and I'm standing at the top. In fact, I've been standing there for some time.
I need to get moving. But - jesus - the slide is almost vertical for the first few metres of its 20-metre drop, before curving sharply and launching racers out into a lake.
I'm standing in silence, considering my options. There's no pressure from the marshall at the top. Nuclear are serious when they say "It's your race". In some other events, I reckon they'd have pushed me by now.
She does mention, delicately, that there's a big group coming up and I probably want to go before they get there. Two more runners appear and immediately slide down as I watch.
There's nothing stopping me from turning around and climbing back down, but I haven't come this far to do 95% of a race. When the next pair of runners arrive, the marshall counts down from three, and I go with them. For some reason it's easier that way.
It's over in a second. We hit with a splash, and are met immediately by two serious-looking guys in drysuits. "Everyone OK? All swimmers?"
I'm already calm, almost fully recovered. It's not the falling, it's the jumping.
"I'm fine. Heights are my problem, not swimming."
If I was expecting criticism, I don't get it. The medics are focused, already watching over the next racers.
But I don't get off as easily as I thought. Later, I scramble up onto another high platform and stop dead. There's no way down except a slippery vertical pole, a metre away from the edge. The waiting marshall sees my Spartan Beast t-shirt and assumes that I'm much better at this than I actually am.
"You know what you're doing?"
"Falling to my death, apparently."
He adjusts instantly, patiently talking me through the process of leaning out to the pole and wrapping my legs around it. Another marshall waits at the bottom, ready to catch me, but I don't need it. The physics are easy; it's just the psychology that's hard.
It happens twice more, with a tall, mud-slick, tilted wall that threatens me with a spine-cracking backwards fall. The marshall gives me a boost up, then - more importantly - helps me figure out how to climb down. And just before the finish, there's a weird, angled pole-slide that looks far more dangerous than it actually is.
This is important because, each time that you fight your fears and win, you get slightly stronger.
(If you lose, they get stronger. Better not to fight than to lose.)
Nuclear Rush gave me four scary encounters with heights in a single race, each with warm, patient and extremely competent support. The next time I face the Death Slide I'll still be scared of it. But I won't be scared for quite as long.