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It might be called black water rafting but after a long and heavy overnight downpour the water rushing through Ruakuri Cave, near Waitomo, is more the colour of coffee with a smidgen of milk.

The stream has risen several centimetres and is surging over a 2m waterfall inside the cave with a full-throated roar.

Angus Stubbs, Black Water Rafting Company's chief instructor, stands on the lip of the falls, somehow impervious to the force of water rushing against his legs, and indicates I should edge away from the cave wall and join him.

I clutch my car tyre tube around my waist like a toddler in a paddling pool. And then, with a good shove from Stubbs, launch into the darkness over any rocks that may lurk at the bottom of the falls. I survive the push, and bob up in the pool below with the rest of the ducklings in the group.

There is a gauge on the wall above the falls and the water level, Stubbs says later, is near the point where rigid safety plans demand rafting in the cave must stop, not as it transpires, because of the force of the water but because of the noise.

If that seems an odd safety rule consider that 500,000 people have jumped off these falls in the 20 years the Black Water Rafting Company has been in business.

It may be an overworked cliche to harp on about New Zealand ingenuity and entrepreneurial inventiveness, but commercial black water rafting is a Kiwi invention that deserves the "legendary" moniker the operation gives itself.

At the 20 year celebration at Ruakuri this month its founders, Pete Chandler and John Ash, wore their New Zealand Order of Merit medals, awarded for their contribution to the tourism industry.

It started in the early 1980s when Chandler, a Waikato University earth sciences graduate and keen caver, was the first to take tourist parties through the underground river running through Ruakuri Cave, a fracture in the limestone created over 30 million years ago from compacted marine debris.

In 1987, Chandler teamed up with enthusiastic caver Ash to establish The Legendary Black Water Rafting Company, since sold by the entrepreneurs to tourism conglomerate, Tourism Holdings. It's actually tubing rather than rafting but you can forgive the hyperbole.

"What began as a special treat for a brave few has developed into a spectacular tourism attraction giving visitors of all ages the chance to weave, jump, and float through a glow worm-studded subterranean wonderland," says Grant Webster, THL chief operating officer.

They come from all over the world to tube the Ruakuri labyrinth, but mostly from Britain and the United States. ShermansTravel.com, a New York-based travel site attracting four million visitors a month, recently included The Legendary Black Water Rafting Company in their Top 10 Extreme Vacations list.

It's bizarre, really. As Grant Davidson, head of the outdoor education and guide training centre, put it at the 20-year celebration soiree; "Who would have thought you could make a business out of making people cold, wet and miserable?"

But as Davidson also noted, an adventure, by definition, is something with an unknown outcome, and from the time you launch yourself into the tiny opening where the Ruakuri Stream disappears underground until the point you reappear into daylight via an equally small opening, there remains the sense of the unknown.

Neoprene wetsuits have replaced woollen pullovers and LCD headlights replaced carbide lamps, but the subterranean river is unchanged.

For me the highlight of tubing through Ruakuri, the "den of dogs," was to link up, feet to feet, in an "eel", and gently float through "caverns measureless to man" beneath an astounding overhead panoply of glow worms. Quiet and magical.

A legend? Most certainly.

- Detours, HoS