The scream team: Britain's ultimate vertical-drop rollercoaster

It's big, it's terrifying and it cost £15m to build. Thirteen is Britain's ultimate vertical-drop rollercoaster. Holly Williams joins the dedicated thrillseekers who are its first victims

It begins with a series of steady climbs and steep descents, swooping curves and sharp corners. Then you enter a dark crypt, and suddenly the whole thing – train and track – drops in stomach-flipping free fall before finally shooting backwards. For some, the thrill is like nothing else. I felt as if I was about to have a heart attack.

This is Thirteen, billed as the ultimate rollercoaster. It has just been unveiled – all £15m worth of it – at Alton Towers, the Staffordshire theme park dedicated to finding new ways to give visitors the ride of their lives.

Thirteen is the world's first vertical free-fall drop rollercoaster, and among fans of such experiences, the excitement has been building for months. Inundated with calls from people begging to be the first to try out Thirteen, Alton Towers decided to run a boot camp to root out the most determined and desperate fans. Over two days earlier this month, 30 rollercoaster obsessives went through psychometric and physical tests, an hour's endurance ride on another rollercoaster – the Air – and late-night challenges in the woods, before results were delivered in a tense X-Factor-style showdown, and 18 of the group learnt they had been chosen.

"People who are really into rides want to be the first," explains Morwenna Angove, Alton Towers' sales and marketing director. "It gives people an amazing buzz. Also, if you're passionate about rides you want to experience something that is new and different."

The ride was designed with input from visitors, so it is fitting that the fans get the first go. They are irrepressibly enthusiastic, but also bring a lot of critical thinking to bear. After the first ride there's a lot of talk of "air time" and "negative G-force" (that would be the feeling that your guts are about to fall out as you descend a particularly steep slope); of hills and "camel humps"; of the importance of "theming".

"It's absolutely amazing," declares Christopher Sawyer, 21, from Wolverhampton. "Worth the hype. It's so smooth, and you get so much air time over the hills." Dan Sadler, 28, from Portsmouth, enthused: "I went nine times, but I could have sat on it all day. I love the first drop, and there's a really nice, weird twist towards the end."

Ride consultant John Wardley came out of retirement especially for Thirteen. He's the man responsible for other Alton Towers world firsts, including the vertical coaster Oblivion, the flying coaster Air, and his particular baby, the standing coaster Nemesis.

"Rollercoasters are very predictable things," Wardley tells me. "We thought, what can we do that has never been done before? If we could have a free-fall drop, where the track itself drops, combined with special effects and the right build-up, it'll be amazing. The mass of the train and the track dropping is enormous. So there are these massive magnetic breaks that bring the track to a safe halt. The ride really will be heavy and intense; but at the same time, fun. I don't want people to come off petrified. Certainly the enthusiasts from boot camp came off laughing and cheering, and that's what a ride is all about."

It's a key point – a ride can't risk alienating the ordinary punter. But a new rollercoaster always proves a draw, and Alton Towers is forecasting a busy year. Theme parks are booming: a quarter of Brits visited one last year, according to market researchers Mintel. Thanks to the rise of the recession-friendly stay-at-home holiday, admissions look set to reach 14 million in 2010. "We started to see the advent of the 'staycation' about 18 months ago," says Angove. "The resort has had a great two years."

While the general public may brave entry charges and queues, the intrepid souls in boot camp go a lot further. From archery and orienteering to erecting a tent in the woods while blind- folded, participants had to prove they really wanted that first ride. Simon King, on hearing he has made it through, says he is "elated, but I don't really know what I've got left in the tank – they've put us through the wringer!"

Melissa Minton,18, certainly isn't put off: "I could happily go on another three or four times." She's with her boyfriend, Lorin James Clark, who tells how they discovered a mutual love of roller- coasters. "She had a school trip to Alton Towers two years ago, but got sent home for drinking. She was sad that she didn't get to go. So for Christmas I got her an annual pass."

Since then they have been regulars: "We've got pictures where we're kissing on rides. It would be definitely good to go on Thirteen together," says Minton. Clark, 21, adds: "We never go to theme parks without each other. It would be the first time we've been on a rollercoaster separately, so that would be weird."

Only Minton makes it through, however, and there are hugs and tears when the result is announced. She tells me afterwards: "I thought, I'm going to be strong, but it all came out and I burst into tears. He was like: 'Just go and enjoy yourself.'"

Clark needn't have worried. "The ride was absolutely amazing," gushes Minton. "Some people look at a load of tracks and a load of parts and just think, what's this about? But because we love the adrenaline rush, it is just so beautiful. It might sound weird, but literally to look at a rollercoaster and go on it and appreciate it is beautiful."

The fun of the fear doesn't diminish with repeats either: the 18 lucky riders were still loving it, even after nine rides (I proudly managed three, and thought my heart was about to give in). "I would stay on all week to be honest," Sawyer tells me. "I'm back on Saturday – and I'll still be at the front of the queue, that's how much I love it."