I check the locked lap bar three times, maybe four. The dusty morning light glints off the coaster cars' snaking track, the smell of resin and popcorn blows through our hoodied crowd, and I, on my virgin ride, fret quietly about where to put my hands.
In front of me, Colin McWilliam, one of the world's most committed thrill-ride enthusiasts and veteran of more than 500 different rollercoasters in nine countries, grins at me. "Make sure you look down," he advises, "to get maximum scared."
Beside him in the front seat, in an embroidered bomber jacket that boasts of his record-breaking naked rollercoaster ride, sits Richard Jones, hands raised in anticipation.
After two days of endurance tests, late-night scares in unlit woods, psychometric testing and communal meals in airy conference halls, they were two of 18 rollercoaster lovers who had passed trials to be the first people on the newest, scariest ride at the Alton Towers adventure theme park in central England.
Builders are still welding the path together outside. Some walls are yet to be built. To our right, a costumed wraith asks for a toilet break.
"This is the best experience of my whole life, ever," whispers Jones, his eyes wide to show he means it. Thirteen is a ride that will "combine the ultimate elements of physical and psychological fear", Alton Towers promises, to create what it calls a "psychoaster".
Based on the discovery of an ancient burial site deep in the Alton woods, its peaks and tracks are invisible from the path.
"I always start by thinking about the palette of emotions," explains its 59-year-old designer, John Wardley, known as "King of Coasters".
In the 1960s, Wardley worked in special effects on James Bond films, shearing the top off a bus in Live and Let Die and making laser weapons for The Man With the Golden Gun.
In 1976, he was invited to "shake up" an ailing zoo near London called Chessington with his animatronics and his plans to let the public experience the thrills of special effects first-hand.
When Chessington's owners, the Tussauds Group, bought Alton Towers in 1990 he started to design the rides (Nemesis, Europe's first inverted coaster in 1994; Oblivion, the world's first vertical-drop coaster in 1998) that would immediately become known as the best in the world.
"With any ride there's thrill, there's exhilaration," he says, leaning close.
"But with Thirteen, we wanted to add enchantment. Mystery! Shock!"
It's so terrifying they'll have to sign a waiver attesting to their mental stability - so disturbing there's both an upper and lower age limit.
Rumours about this ride's groundbreaking thrills have been picked over online for months. Fans have found the blueprints, builders have leaked early photos. Will it throw riders into open graves?
As its first riders, we're about to find out. Off to the rear of the train, two ride operators in high-visibility vests press their On buttons in unison, and our cars begin to slide slowly up a teetering hill. As we climb, I hold the lap bar and try not to think of death.
Alton Towers began its slow evolution from stately home to theme park in 1980 with the installation of the Corkscrew, now the site of Thirteen. Its rides, Wardley tells me, are currently at the top of international rollercoaster fan clubs' top 10s.
These days, rollercoasters compete to break world records for height, for speed, for loops, for drops.
Their parks are holiday destinations with hotels, spas, restaurants. And they continue to generate profit; in 2005, 20 million people visited British amusement parks.
Colin McWilliam, a bricklayer who lives alone in his fairground-themed apartment, once found romance on a rollercoaster: "We made eye contact in the queue," he grins, "and swapped numbers in the exit shop."
"I wish I could get bored of rollercoasters so I could get on with my life," he says.
"But I know me. I know I won't." On his left arm he has a tattoo of Disneyland Paris' Space Mountain, and on the right, 10 years old, the Alton Towers' coat of arms.
McWilliam's website, UKRides.info, last year received more than 50 million hits. "I couldn't tell you how it began, this obsession," he says.
"It's just in me. When I was a kid my parents put me on the roundabout in the local market and I refused to get off." For Nicola Pickford, one of Britain's few female thrill-ride enthusiasts, it is "all about the feeling".
She describes the experience of riding a rollercoaster, the fragile illusion of danger, in loose metaphors and gasps. Her mother wouldn't let her go to amusement parks, but when she visited Disneyland Paris on a school trip, "it changed me - it brought out my wild side.
"I don't care about the structure, you know, the bolts. I just want my bones shaken."
As we fall from Thirteen's summit, my thighs pressed tight against the bar, I scream for the first time.
Screaming helps as we spin through the forest; for a second I'm reminded fondly of a car crash I was once in, but I leave any real thoughts somewhere behind me, on the second hump perhaps, when I start to almost love the feeling of dropping a belly up by the branches of an oak tree, and I'm somewhere between laughter and shriekery as we reach the half-built crypt.
In the pitch black a pause, then the ground beneath our track seems to crumble before suddenly giving way completely.
We drop, like screaming stones. When the rollercoaster finally grunts to a halt a minute later, there's exhausted applause.
"I feel high," says McWilliam.
"It's like seeing a horror film at the cinema, but being inside it, too." I climb on to land, wobbly, elated, and surprised, somehow, to have arrived back exactly where I began.
SEEKING THRILLS FROM LOGS TO LOOPS
The typical rollercoaster enthusiast is, says psychologist Dr Frank Farley, "a thrill-seeker" who looks for "variety, novelty, intensity and risk".
Another psychologist, Marvin Zuckerman, proposes that rollercoaster lovers have an imbalance in the brain chemical monoamine oxidase, which is also implicated in depression.
Excitement changes the level of the chemical, lifting them, briefly.
Farley concludes that thrill seekers have a neurological need for excitement, that their brains are at a lower level of arousal than the average person, and that the physical extremes of a rollercoaster ride - the drops, the speeds, the rickety climbs - prime the reticular activating system, a neural network at the base of the brain, which then heightens the level of activation.
To be scared makes them feel more alive.
The earliest rollercoasters were ridden in 15th-century Russia - sleds made of hollowed-out tree trunks, sent down man-made ice-covered hills at 80km/h. It wasn't until the 19th century, though, that the Americans worked out how to make rollercoasters pay.
The first "switchback gravity railway", Mauch Chunk in Pennsylvania, was built in 1827 to haul fuel.
Trains loaded with anthracite coal sped down a 9-mile mountain - the afternoon runs along Gravity Road carried thrill-seeking passengers, each paying 50 cents a ride.
Sixty years later they built one just for pleasure at Coney Island; it was followed by a looping railway, and then a scenic ride through theatres of well-lit scenery, biblical tableaux and artificial forests.
The world's tallest ride is Kingda Ka in New Jersey, struck by lightning in 2009. The longest (Steel Dragon 2000, in Mie, Japan, a rollercoaster that cost US$50 million to build) will - with magnetic brakes, technology only developed in the past two years - make riders feel quite odd.