"TOM CRUISE was here a few weeks ago," says my guide Gummi as we peer into a 400ft black hole. "He went down into the volcano but then insisted he wanted to do it again, with only a rope!" This summer, Iceland's Thrihnukagigur volcano has opened up to tourists for the first time, allowing them to descend into one of the world's only accessible magma chambers in a metal lift.

The Mission Impossible star, on location here for his new film Oblivion, cancelled his second exploration at the 11th hour and flew home to Los Angeles after his wife Katie Holmes filed for divorce.

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Today I'm one of a small group of tourists following in his adventurous footsteps, although as I peek into the gaping hole I'm glad we're not going down via rope alone.

After a one-hour drive south-west of the capital Reykjavik and a 45-minute ascent over a lava field, covered in lichen-coloured boulders under a chalky grey sky, we arrive at the entrance to the underground chamber.

It's a mid-summer's day but windproof jackets are essential as the elements howl and screech around us. I can see why Hollywood chose Iceland as a location for the action sci-fi film released next year about a soldier sent to destroy the remains of an alien race.

There's certainly something otherworldly about this northern country sculpted by fire and ice and famed for its dramatic landscape of geysers, waterfalls and hot springs.

Meanwhile, hip Reykjavik, the world's most northerly capital, has long been a by-word for cool bars and nightclubs.

The only way down into the chamber is via a tiny cradle - similar to those window cleaners use to clean skyscrapers - attached to a gantry across the entrance. We don hard hats and harnesses. It's all pretty low-tech and, divided into groups of four, we exchange nervous looks as the lift whirs into life and lowers us into the abyss.

As the chamber opens up the sound of the screaming winds fade away. The subterranean space opens up around us as floor lights reveal a sm¶rg¥sbord of colour.

Bedrock, cooked at 1,000C plus temperatures during the last eruption 4,000 years ago, has cooled into a m©lange of oranges, reds and browns.

When we reach the bottom, the cradle lifts away leaving us alone to scramble around the playground of boulders. We're told not to venture far. This is pure raw exploration in a land still being torn and renewed by the tectonic plates which grind against each other. "Usually you can't get into volcanos as either they're full of lava or they've erupted meaning all you have left are craters," Gummi explains.

Thrihnukagigur is considered by experts to be extinct - though nobody can be absolutely sure.

During its last eruption the molten lava left a bottle-shaped hole. The physics of this meant the volcano didn't naturally collapse in on itself and a pressure drop drained the magma away leaving a perfectly preserved chamber that was only discovered in 1974.

After an hour the cradle inches down to return us, grubby and elated, to the surface. The experience can't be replicated anywhere else on Earth.

Perhaps one day Tom Cruise will return with that rope and be able to fulfil his dream of going underground again.

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THE KNOWLEDGE

WOW AIR (dialling from the UK: 00 354 590 3000/www.wowair.com) offers return flights from Stansted to Reykjavik from £141. Fosshotel Lind (562 4000/www.fosshotel.is) offers doubles from £67 per night (two sharing), B&B.

Inside the Volcano (863 6640/www.insidethevolcano.com) offers daily tours of the volcano from £186pp. Available until August 20.

Iceland Tourism: www.visiticeland.com