Lord of the rubber rings

All around me is silent, except for a mild whoosh of water. I'm peacefully daydreaming, floating on a large rubber inner tube, and it's pitch black, save for an amazing Milky Way of lights dotted far above my head. But I'm not bobbing along on the ocean on a summer's eve and the lights above are not stars. I'm 180ft below ground in the freezing water of an underground river, and the lights are the iridescent innards of glow-worms.

I'm blackwater rafting in New Zealand's famed Waitamo cave system. There are myriad caves in the area. Some, such as Glow-worm Cave, are easily accessible by foot and are visited by hordes of tourists. But the the cave I'm in now - Ruakuri Cave - is deep underground and inaccessible without mountaineering equipment.

The licence for caving in Ruakuri is retained by a company called Blackwater Rafting and owners Peter Chandler and John Ash were the first to introduce cave tubing to the region in 1987. If any company is going to know how to give you a thrill, it has to be them.

There is only one way to get into the cave - abseiling down a shaft. Our guides are Lovely Lisa Bourke and Mad Steve Mahoney, who kit us out in wetsuits, helmets and harnesses before taking us to a training ground next to the cave entrance.

Steve pulls the usual tour-guide gags about losing people and the freezing conditions we're going to encounter, but the training is deadly serious. After an hour's instruction, we are ready to go.

Lisa is the first to go into the unknown, closely followed by me. The descent sends the adrenalin coursing. I can't see the bottom. Lisa begins to sing and her words echo up the shaft, lending a magical quality to the descent.

My four colleagues and Steve follow us to the bottom and we descend further into the cave system on foot. Lisa tells us that the cave we're in has an offshoot that was a Maori burial site and that, when visiting it on her own, she's often felt she wasn't alone.

Suitably spooked, we proceed over a chasm and on to the ledge from which we will enter the water. There's little glamour involved, thanks to the outrageous kit we're wearing - wetsuit, baggy shorts, rubber bootees and red miners' helmets - and even less grace. You simply grab an inner tube, stick it under your backside, and leap into the unknown. Hitting the water sucks the breath right out of you, and the huge splashes make the worms glow more fiercely. (The light is their poo and the sound scares the s**t out of them.)

As we paddle slowly through the cave, it's not hard to imagine this being the Mines of Moria that Gollum thrived on in The Hobbit. It takes us about an hour to travel the mile or so to the end of the tunnel and, upon reaching it, we return back down the same route, legs linked, lights out, being pulled by a singing Lisa.

We've now been in the cave for three-and-a-half hours and the water is starting to chill to the bone: Steve says that without wetsuits, we'd have got hypothermia in 20 minutes. We stop for chocolate and begin our ascent, climbing through two steep waterfalls to the surface.

Above ground, the sun shines brightly. But it's the sublime sense of satisfaction that really thaws us.

Way to go

The five-hour Blackwater Rafting II trip costs £57. Tel: 0064 7 878 6219, www.blackwaterrafting.co.nz Waitamo Glow-worm Cave. Tel: 0064 7 878 8227, www.waitamocaves.co.nz James flew to Auckland with Air New Zealand (Tel: 0800 028 4149, www.airnewzealand.com) and stayed at the Cambridge Mews motel, which is an hour's drive from the caves (Tel: 0064 7 827 7166, www.cambridgemews.co.nz).