Are you feeling scared yet?

On Saturday, Alton Towers unveils its new attraction for thrill-seekers, Nemesis Sub-Terra. Simon Calder gets a sneak preview of what's billed as an underground nightmare and finds that he's not as tough as he thought

Feel the fear factor: Simon Calder prepares to take the Nemesis Sub-Terra ride

The pun is intentional: Nemesis Sub-Terra is designed to instill terror in the thousands of riders who, from Saturday, are expected to surge through the turnstiles of Alton Towers. But how scary is the new ride? At least two of us are keen to find out: me, and my new friend, Dr Paul Tennent, whom I have known for only five minutes but already has my shirt off and is attaching electrodes to my chest.

Dr Tennent's day job is as a computer-science lecturer in Nottingham. But he is also part of the Thrill Laboratory. This is a collective of scientists, psychologists, sociologists and architects "dedicated to the practical pursuit of creating, producing and examining new forms of thrilling experience", which is just what Merlin and I need.

Merlin is the UK's biggest theme-park company. It runs attractions from the London Eye to the Blackpool Tower, but its main focus is on what it might like to call the triumvirate of terror: Thorpe Park and Chessington World of Adventures, both south-west of London, and Alton Towers in northern Staffordshire. These theme parks seek to provide big thrills on an industrial scale, creating an addiction to adrenalin that keeps the crowds coming. The best way to feed that habit is to reinvent themselves even more frequently than Madonna and install a big new ride for the start of a season.

Nemesis Sub-Terra is Alton Towers' bid for 2012. Building for what was code-named the Catacomb Project began in September. The new ride is described as "your worst nightmare underground". But just how thrilling is it? That is what Dr Tennent and I are hoping to find out; it explains why he is connecting devices to my fingertips to detect stress in the form of micro-beads of sweat – the same principle used in lie-detector technology, honest. "We want to collect a caucus of data about the physiological changes induced by thrills and the different ways people respond," he says. The organisation identified an ideal gene pool of expendable guinea pigs and six journalists turned up on Monday for testing.

The attraction is related to Nemesis – the roller coaster that transformed Alton Towers into a premier-league attraction when it opened in 1994. While the gaunt, serpentine structure of Nemesis makes clear you can expect a ride where your body, soul and stomach simultaneously depart in different directions, its Sub-Terra sibling gives no clues. The venue is a big steel shed, painted military green and looking remarkably like some of the more utilitarian airport terminals Ryanair uses at former air force bases in Germany.

And the shadowy paramilitary force guarding the place is fiercer than Ryanair cabin crew. They wear black and shout a lot. Indeed, you might think they are rejects from the academy that used to train East German border guards. They are called The Phalanx.

Dr Tennent is now measuring my breathing patterns. He is not like other computer-science lecturers: he spent his honeymoon in Disneyland Paris. And his fellow researcher, Dr Joe Marshall, who is helping to rig the infrared camera on a frame on my chest, once held the record for 24-hour all-terrain unicycling – a superlative of which I was unaware.

The more scientists understand about fear, the better they can define who is a suitable candidate for rides like Nemesis Sub-Terra. The 1.4m height restriction helps to sort out the men from the boys (it is a safe bet that males will dominate the clientele). But for the first time, the British Board of Film Classification has been brought in to rate a theme-park attraction.

After several tests, Murray Perkins, senior examiner at the BBFC, classified it as a 12A. In other words, under-12s are not allowed in unaccompanied, although "an adult may take a younger child if, in their judgement, the film is suitable for that particular child".

Over 11? Check. Over 1.4m? Check. All plugged in? Time to play.

As with queuing for a Ryanair flight, some passengers are more equal than others. After standing in line with the masses, cutting through in the fast-track lane or joining the loners in the Solo Riders Only queue (that's me), you enter the structure. Doors open and you are ushered – no, ordered – into a lift. The noises, lights and vibrations suggest you are descending deep into the ground.

You emerge into a chamber that feels like the lair for a low-rent James Bond villain. The tableau at the centre is surrounded by 1960s sci-fi zappers, pointing at a giant egg that you are led to believe will hatch at any moment, with unpleasant consequences. More shouting directs you to a seat, where the "sequesters" – the usual theme-park restraining harnesses – are lowered. Then the sudden darkness indicates the experience is about to begin.

A sharp spray of air hits you in the face. You drop into the void. You jolt to a halt and as you try to make sense of your surroundings a tentacle or two jabs you from behind. Then the guards come into their own. Like extras from a hostage-simulation exercise, they yell at you to escape. Seconds later you find yourself back in leafy Staffordshire. And, in my case, being untangled by Dr Tennent.

"The data maps what the ride is doing," he says. The breathing record shows an involuntary gasp when the lights went out. After the blast of air, I started shallow breathing – though Dr Tennent was kind enough to describe it as "manly panting". The best correlation, though, was with the heart monitor. "You're generally a flatliner, but when the ride dropped your heart rate jumped from about 60 to 172 – and then again when you were poked in the back of the legs," he says. "The ride is going for that sudden intensity – as though you went from normal to running full-tilt in an instant." The highest rate was saved for the end when the shouting reached a peak. It shot up again to 186 – three times the rate at rest. Evidence, perhaps, of a latent fear of authority or simply too many encounters with East German frontier officials. It shows the best thing about the ride is its theatricality.

The BBFC's guidelines for a 12A allow "moderate physical and psychological threat, provided that the disturbing sequences are not frequent or sustained". I suspect most people who pass the 12 years/1.4m hurdles and try Nemesis will latch on to that word "moderate".

We do not go to theme parks for "moderate" experiences. We go to be scared witless. I plan to be first in the queue for The Swarm, the new ride at Thorpe Park. It promises "a death-defying flight through apocalyptic devastation", which doesn't sound too moderate, with "near misses and gut-wrenching inversions as it rips through the sky on its mission of complete annihilation". You don't get that on Ryanair.

As the sun sinks over Staffordshire, the scientists are studying my heart-rate graph, which shows peaks even Dr Marshall might hesitate to unicycle down and tell a different story to my conscious response to Nemesis Sub-Terra. "It shows that it doesn't matter how calm you are – the ride will scare the crap out of you," Dr Tennent says. Is the purpose of the exercise, I wonder, to snigger at what scaredy-cats journalists are? No, it is much more important than that. "If you understand how people respond to thrills, you can design better rides," Dr Tennent says.