The taxi driver taking us from Inverness looks dourly towards the mountains surrounding Scotland's most famous stretch of water. 'Do I believe in the Loch Ness monster?' he asks, rhetorically. 'Fervently. It's great for business.'

Indeed. Since 1 April this year, more than 700 people have travelled to the Highlands and paid pounds 68.50 to search the waters aboard Loch Ness One, the country's first public submarine.

'My own view is that there might be something down there,' says Alan Whitfield, pilot and owner of Loch Ness One. 'Why not? As a diver, I've come across so many odd things in other parts of the world. And it would do us no good to, um, deny the existence of the Loch Ness monster completely.

'We'll descend 500 feet to the plateau in the middle of the loch,' announces Mr Whitfield, at the beginning of the hour-long dive. 'Then we will fly over the surface of the plateau.' The five-person 'crew' comprising visitors from Barnsley and Weston- super-Mare, regard him anxiously. We are sitting in a somewhat nervous fashion on benches within the 34ft submarine; sunlight filters through the large 'viewport' as we prepare for the dive.

'The trip to the bottom of the loch is about the same as falling from the top of the Post Office Tower,' continues Mr Whitfield. 'It is the largest single expanse of fresh water in Europe.'

As little as 30 feet down, the light turns from pale green to murky black as we drop, tail first, through the icy depths of the loch. Large bubbles pass the window. A twanging sonar bleep lends Cousteau-like credibility to the atmosphere.

'This is the most dangerous moment. Vents open]' cries Mr Whitfield on a walkie-talkie. 'Air goes out, water comes in. We'll sink like a stone,' he explains, twiddling with an astonishing array of knobs. Mr Whitfield is dressed in a natty red suit with blue piping. He is slightly reminiscent of a Thunderbirds puppet. 'If I should pass out, push this lever. It's the Emergency Blow Station. It'll send you up to the top in five minutes.'

'I didn't see the lever,' hisses Mary Dodds, one of the Weston-super-Mare party. She is clutching a grinning woollen Nessie, complete with knitted tam-o'-shanter, which she has brought specially for the trip. 'He's called Humphrey,' she says. 'Never mind,' returns her friend Peter Hepple grimly. He has gone rather pale. 'This reminds me of being in a Second World War film. Except it's reality.'

'Two hundred feet down,' sings Mr Whitfield gaily. 'This is the maximum depth to which German U-Boats would go. Any deeper, that's when you see the jets of water coming in and the rivets coming out.'

Fortunately, Loch Ness One is made of sterner stuff. We continue on down. 'Holy shit, Mary,' says Mr Hepple. They have driven up from Weston to Scotland specially for this dive. 'She made me come with her. Now I realise why no one else would. I was conned,' he says. 'I suppose they say that drowning's the best way to die.' Ms Dodds waves Humphrey at Mr Whitfield. 'Can't we see some fish or anything?'

It would appear not; at 500 feet underwater, the only evidence of life in Loch Ness is a mass of plankton. 'Anything else down here would be squashed flat because of the water pressure,' says Mr Whitfield, knowledgeably. 'What we're looking at is the food chain.' One by one, we are allowed to climb up to the observation turret and peer out from portholes through the black, peaty water, which is all there is to see. The woman from Barnsley asks if there is a periscope. Mr Whitfield gives her a funny look.

Thirty thousand watts of light from the submarine illuminate the bottom of the loch; however, there is a visibility of only about 15 feet. We realise that the likelihood of seeing a large dinosaur-type animal swimming past is fairly slim, and concentrate instead on the gravelly floor of the loch. 'This is an underwater mountain,' says Mr Whitfield, as if to make up for the absence of anything else.

Ms Dodds nods her head sagely and peers through another viewport built into the floor. 'I just had to come on this trip, as soon as I heard of it. I'm a real believer in the monster,' she confides. 'But this is the first time I've been to Scotland. I've even brought my passport with me. I thought I might need it.'

The 24-ton Loch Ness One, built in Canada 15 years ago, is really a Deep Submersible Rescue Vehicle, capable of carrying up to 27 people (it is only insured for a capacity of six in the loch), and designed to come to the aid of larger submarines.

It was used by the Royal Navy until Mr Whitfield had the bright idea of buying it, stuffing it with tourists and dropping it six times a day to the bottom of Loch Ness. 'I really can't remember how much I bought it for,' says Mr Whitfield, brandishing a perfect smile.

'But of course the main reason for diving here is our scientific programme. We are carrying out a range of important experiments during each dive. Sedimentation studies, for example. In fact, this vehicle is constantly grabbing data from the surroundings.' So, presumably, details like the Cousteau sonar 'ping' are all in the pursuit of science? 'Ah, well, that's actually a

recording,' he confesses. 'Hype. We like to make the atmosphere more, um, realistic to the visitors. I call it educational tourism.'

Even without the monster most people seem to enjoy the experience, although Mr Whitfield's partner, Graham Swindell, did recount tales of claustrophobic Japanese who became 'physically ill' and one American woman who had to be spoken to 'quite severely' when she had a panic attack.

We rise to the top with astonishing speed. Mr Whitfield apologises for the lack of sightings on what his advertising blurb calls 'The Ultimate Monster Hunt', but his 'crew' all look delighted. 'I feel incredibly brave. Incredibly,' says Peter Hepple. 'But I can't wait to get back to England. Bloody boring place, Scotland.' We cruise up to the quay where another little group of Nessie fans stand, waiting for their turn to make history.

'There's a prize of pounds 10,000 from William Hill if Nessie is captured on film,' says Mr Whitfield. 'Entry is included in your ticket.' Ms Dobbs waves Humphrey out of the observation turret at the next group. It is clear her belief in the monster is unabated; it has probably been reinforced.

Around the quayside, promotional literature talks up 'the mysterious phenomenon' of Loch Ness, as if Nessie were a scientific probability rather than a fictional animal as likely to be spotted as a deep-sea diving abominable snowman.

Mr Whitfield has his own explanation for the submarine's on-going failure. 'If you imagine every person in the world could stand in Loch Ness without one head breaking the surface of the water,' he says, 'it's not surprising we haven't yet found Nessie. Anyway, twice we've picked up strange noises at about 600 feet down. They were exactly the same frequency as whale or dolphin sounds. But there are certainly no whales or dolphins here.'

He looks out over the uncompromising surface of Loch Ness. 'Something alive in there has twice talked to us.' And he doesn't mean the bank manager.

The Loch Ness Submarine, Clansman Marina, Drumnadrochit, 0456 450709. Advance bookings required.

(Photographs omitted)