Every country is like a particular type of person. America is like a belligerent, adolescent boy, Canada is like an intelligent, 35 year old woman. Australia is like Jack Nicholson. It comes right up to you and laughs very hard in your face in a highly threatening and engaging manner. In fact it's not so much a country as such, more a sort of thin crust of semi-demented civilisation caked around the edge of a vast, raw wilderness, full of heat and dust and hopping things.

Tell most Australians that you like their country and they will give a dry laugh and say 'Well, it's the last place left now isn't it?', which is the sort of worrying thing that Australians say. You don't quite know what they mean but it worries you in case they're right.

Just knowing that the place is lurking there on the other side of the world where we can't see it is oddly unsettling, and I'm always looking for excuses to go even if only to keep an eye on it. I also happen to love it. Most of it I haven't even seen yet, but there's one place that I've long wanted to revisit, because I had some frustratingly unfinished business there.

And just a few weeks ago I suddenly found the excuse I'd been looking for.

I was in England at the time. I could tell I was in England because I was sitting in the rain under a wet blanket in a muddy field listening to some fucking orchestra in a kind of red tent playing hits from American movie soundtracks. Is there anywhere else in the world where people would do such a thing? Anywhere? Would they do it in Italy? Would they do it in Tierra del Fuego? Would they do it on Baffin Island? No. Even in Japan where national pastimes include ripping out your own intestines with a knife, I think they would draw the line.

In between the squalls of rain and trumpets I fell into conversation with an engaging fellow who turned out to be my sister's next door neighbour up there in Warwickshire, which was where the sodden field was. His name was Martin Pemberton and he was an inventor and designer. Amongst the things he had invented or designed, he told me, were various crucial bits of tube trains, a wonderful new form of thinking toaster and also a Sub Bug.

What, I asked politely, was a Sub Bug?

A Sub Bug, he explained, was a jet-propelled underwater buggy sort of thing. A bit like the front half of a dolphin. You hold on to the rear and it pulls you through the sea at depths of up to thirty feet. Remember that bit in the movie of Thunderball? A bit like those things. Great for exploring coral reefs.

I'm not sure if that's exactly what he said. He may have said 'azure sea' or 'limpid depths'. Probably not, but that was the picture in my brain as I sat in the blustery rain watching an escaped umbrella totter past the bandstand.

I had to try one. I said so to Martin. I may even have wrestled him to the ground and knelt on his windpipe, everything was a bit of a blur to be honest, but anyway he said he would be delighted to let me try one. The question was where? I could try it anywhere, even just in the local swimming pool. No. The trick was to get to try it in Australia, on the Great Barrier Reef. I needed an angle, though, if I was going to get some hapless magazine to stump up a trip for me to try it which, believe me, is the only way to travel.

Then I remembered my unfinished business in Australia.

There's an island I had visited briefly once ten years ago in the Whitsundays, at the southern end of the Reef. It was a pretty dreadful place, called Hayman Island. The island itself was beautiful but the resort that had been built on it was not and I had ended up there by mistake, exhausted, at the end of an author tour. I hated it. The brochure was splattered with words like 'international' and 'superb' and 'sophisticated', and what this meant was that they had Muzak pumped out of the palm trees and themed fancy dress parties every night. By day I would sit at a table by the pool getting slowly sozzled on Tequila Sunrises and listening to the conversations at nearby tables which seemed mostly to be about road accidents involving heavy goods vehicles. In the evening I would retire woozily to my room in order to avoid the sight of maddened drunk Australians rampaging through the night in grass skirts or cowboy hats or whatever the theme of the evening was, while I watched Mad Max movies on the hotel video. These also featured a lot of road accidents, several of which involved heavy goods vehicles. I couldn't even find anything to read. The hotel shop only had two decent books and I'd written both of them.

On one occasion I talked to an Australian couple on the beach. I said "Hello, my name is Douglas, don't you hate the Muzak?" They said they didn't as a matter of fact. They thought it was very nice and international and sophisticated. They lived on a sheep farm some 850 miles west of Brisbane where all they ever heard, they said, was nothing. I said that must be very nice and they said that it got rather boring after a while, and that a little light Muzak was balm to them. They refused to go along with my assertion that it was like having Spam stuffed in your ears all day and after a while the conversation petered out.

I made my escape from Hayman Island and ended up on a scuba diving boat on Hook Reef where I had the best week of my life, exploring the coral, diving with a wild variety of fish, dolphins, sharks and even a minke whale.

It was only after I had left Hayman Island that I heard of something really major that I had missed there.

There was a bay tucked round on the other side, called Manta Ray Bay, that was full, as you might expect, of manta rays: huge, graceful, underwater flying carpets, one of the most beautiful animals in the world. The man who told me about it said that they were such placid and benign creatures that they would even allow people to ride on their backs underwater.

And I had missed it. For ten years I fretted about this.

Meanwhile I had also heard that Hayman Island itself had changed out of all recognition. It had been bought up by the Australian airline Ansett, who had spent a squillion dollars on ripping the Muzak out of the palm trees and transforming the resort into something that was not only international and superb and sophisticated and so on, but also breath-takingly expensive and, by all accounts, actually pretty good.

So here, I thought, was the angle. I would write an article about taking a Sub Bug all the way to Hayman Island, finding a friendly manta ray and doing, effectively, a comparative test drive.

Now any sane, rational person might say that that was a thoroughly stupid idea, and indeed a lot of them did. However, this is that article: a comparative test drive between an underwater propeller-driven, blue and yellow one person Sub Bug, and a giant manta ray.

Did it work out?

Guess.

The sheer fatuous unreality of the idea struck me forcibly as we watched the huge 40 kg silver box containing the Sub Bug being wheeled across the tarmac at Hamilton Island airport. There was, I realised, a huge difference between telling people in England that I was going to Australia to do a comparative test drive between a Sub Bug and a manta ray, and telling people in Australia that I had come to do a comparative test drive etc. I suddenly felt like an extremely idiotic Englishman whom everyone would hate and despise and point at and snigger about and make fun of.

My wife, Jane, calmly explained to me that I always became completely paranoid when I had jet-lag, and why didn't I just have a drink and relax.

Hamilton Island looks like a pretty good example of what not to do to a beautiful sub-tropical island on the edge of one of the great wonders of the natural world, which is to cover it with hideous high-rise junk architecture, and sell beer and T-shirts and also picture postcards of how beautiful it used to be before all the postcard shops arrived. However we would only be there for a few minutes. Sitting waiting for us at the jetty by the small airport was the Sun Goddess, which was the sort of glamorous gleaming white boat that James Bond always seemed to spend an inordinate amount of time on, considering he was actually supposed to be a civil servant. It had been sent to meet guests going on to Hayman Island, and was the first indication of how much the place had changed.

We were ushered graciously aboard. One attendant offered us glasses of champagne while another stood guard by the sliding glass doors which led into the air-conditioned interior.

His job was to push them open for us. He explained that this had become necessary because unfortunately the doors didn't open automatically when you approached them, and some of their Japanese visitors would often just stand in front of them for whole minutes getting increasingly bewildered and panic-stricken until someone slid them open by hand.

The journey took about an hour, streaking effortlessly over the dark and gleaming sea under a brilliant sun. Smaller lush green islands slid past us in the distance. I watched the long wake of water folding back into the sea behind us, sipped at my champagne and thought of an old bridge that I know in Sturminster Newton in Dorset. It still has a cast iron notice bolted to it that warns anybody thinking of damaging or defacing the bridge in any way that the penalty is transportation. To Australia. Now Sturminster Newton is a lovely town, but it astonishes me that the bridge is still standing.

Jane, who is much better at reading guide books than me (I always read them on the way back to see what I missed, and it's often quite a shock) discovered something wonderful in the book she was reading. Did I know, she asked, that Brisbane was originally founded as a penal colony for convicts who committed new offences after they had arrived in Australia?

I spend a good half hour enjoying this single piece of information. It was wonderful. There we British sat, poor grey sodden creatures, huddling under our grey northern sky that seeped like a rancid dish cloth, busy sending those we wished to punish most severely to sit in bright sunlight on the coast of the Tasman sea at the southern tip of the Great Barrier Reef and maybe do some surfing too. No wonder the Australians have a particular kind of smile that they reserve exclusively for use on the British.

From offshore, Hayman Island looks deserted, just a large verdant hill fringed with pale beaches set in a dark blue sea. Only from very close to can you spot the long low hotel nestling among the palms. There is hardly anywhere you can get a good look at it from, since it is virtually smothered with what look like giant feral pot plants. It snakes and winds its way through the greenery: pillars, fountains, shaded plazas, sun decks, discreet little shops selling heart-stoppingly expensive little things with designer labels you'd have to carefully unpick, and indiscreetly large swimming pools.

It was pretty fabulous. We adored it immediately. It was exactly the sort of place that twenty years ago I would have despised anybody for going to. One of the great things about growing older and getting things like freebie holidays is that you can finally get to do all those things that you used to despise other people for doing: sitting around on a sundeck wearing sunglasses that cost about a year's student grant, ordering up grotesque indulgences on room service, being pampered and waited on hand and foot by - and get this, this is a very important and significant part of what happens to you on Hayman Island - staff who don't just say "No worries" when you thank them for topping up your champagne glass, they actually say "No worries at all". They truly and sincerely want you, specifically you, not just any old fat git lying around in a sun hat, but you personally, to feel that there is nothing in this best of all possible worlds that you have come to for you to concern yourself about in any way at all. Really. Really. We don't even despise you. Really. No worries at all.

If only it were true. I had my Sub Bug to worry about, of course. This huge great thing that I had lugged ten times further than Moses had dragged the children of Israel, just in order to see how it compared with a manta ray as means of getting about underwater. It had been quietly removed from the boat in its huge silver-coloured box and discreetly stored at the dive centre where nobody could see it or guess at its purpose.

The phone rang in our room. The room was extremely pleasant, incidentally. I'm sure you're keen to hear what the room was like since we were staying in it at your expense. It was not enormous but it was very comfortable and sunny and tastefully decorated in Californian pastels. Our favourite item was the balcony which overlooked the sea because it had an awning which you lowered by pressing an electric switch. The switch had two settings. You could either turn it to "AUTO" in which case the awning lowered itself whenever the sun came out, or you could set it to "MANUEL" [sic] in which case, we assumed, a small incompetent Spanish waiter came and did it for you. We thought this was terribly funny. We laughed and laughed and laughed and had another glass of champagne and then laughed some more and then the phone rang.

'We have your Sub Bug,' said a voice.

'Ah yes,' I said. 'Yes, the, er, Sub Bug. Thank you very much. Yes, is that all right?'

'No worries,' said the voice, 'at all.'

'Ah. Good.'

'So if you like, why don't you come down to the dive centre in the morning. We can check it out, see how it works, see what you need, take it out for a spin, whatever you want. We'll just do whatever we can to help you.'

'Oh. Thank you. Thank you very much.'

'No worries at all.'

The voice was very friendly and reassuring. My jet-lagged paranoia began to subside a little. We went and had dinner.

The resort had four restaurants, and we chose the Oriental Seafood Restaurant. Seafood in Australia mostly seems to consist of barramundi, Morton Bay bugs and everything else.

'Morton Bay bugs,' said our smiling Chinese waitress, 'are like lobsters only this big.' She held her two forefingers about three inches apart. 'We smash their head in. Is very nice. You will like.'

We didn't like that much in fact. The restaurant was very smartly decorated in Japanese style black and white, but the food looked better than it tasted and they played Muzak at us. For a moment I felt the ghost of the old, naff Hayman Island stalking through its glamorously tasteful new home. The other restaurants available were Polynesian, Italian, and the restaurant to which they gave top billing, 'La Fontaine', a French restaurant which we decided to keep for the last of our four nights, though we had nagging doubts. I tend to like local cooking unless I'm in Wales, and the thought of French haute cuisine transported here did not fill me with confidence. I wanted to keep an open mind, though, because as it happens one of the best meals I ever had was steamed crab and chateaubriand of zebu cooked by a French-trained chef in the south of Madagascar. But then the French had infested Madagascar for 75 years and bequeathed it a rich legacy of culinary skills and hideous bureaucracy. We decided at least to look at La Fontaine that night. As we prowled our way towards it, we traversed acres of beautifully laid carpet, passed grand pianos, chandeliers and reproduction Louis XVI furniture. I found myself racking my brains for any memory I might have of perhaps some schismatic 18th Century French court that might have been set up, however briefly, on the Great Barrier Reef. I asked Jane, who is an historian, and she assured me that I was being extremely silly, and so we went to bed.

We were woken at precisely seven-thirty the following morning and indeed, every morning by a seagull which perched on our balcony and performed our regular early morning wake-up screech. After breakfast we went to the dive centre, which was about half a mile from the hotel, and met Ian Green.

It was Ian who had called the previous evening. He was in charge of all the diving stuff on Hayman and a more helpful and friendly person would be hard to imagine. We got the Sub Bug unpacked and examined it as it stood gleaming in the sun.

It is, as I have said, shaped like the front half of a dolphin. The body of it is blue, and towards the front there are two small yellow fins, one on each side, which can rotate through a few degrees and direct the Sub Bug upwards or downwards. At the back are two large handles which you hold on to as the Sub Bug pulls you through the water. Within reach of your thumbs are buttons which make the thing go, and control its ascent and descent. Inside the bug is a cylinder of compressed air - a normal scuba cylinder - and this provides power to spin the two propellers which push the bug forward, and also supplies air down a flexible tube to a free-floating regulator. A regulator is the thing you stick in your mouth which gives you your air when you're diving. The point of this arrangement is that you only need your mask and flippers. You don't need to carry a scuba tank on your own back, because you're getting your air direct from the Sub Bug. The Bug is designed in such a way that you can set a maximum depth beyond which it will not go. The very maximum anyway is 30 feet.

Ian had received a flurry of faxes from Martin Pemberton about setting up the machine, and was pretty confident about it.

'No worries at all,' he said, and asked me what I planned.

I said it might be an idea to take it for a shallow local try-out before taking it out into deep water.

'No worries,' he said.

I said that we could then take it with us on the proper diving expedition that was going out from the island the following morning.

'No worries,' he said.

'So I will then spend a little time trying it out, getting used to it and putting it through its paces around the reef.'

'No worries,' he said.

'And then, er,' I said, 'for the purposes of this article I have to write, which is by way of being a sort of comparative test drive, I want to try the same thing on a manta ray.'

'No chance,' he said. 'No chance at all.'

I suppose I should have foreseen this.

Or perhaps it was just as well that I hadn't foreseen it. If I had foreseen it I wouldn't have been standing there half in a wet suit looking out at the glittering Tasman Sea and thinking 'Oh damn.' I would have been sitting fiddling in my office in Islington wondering if I'd done enough 'work' yet to justify going out and getting a bun.

The issue was very simple. As someone who has spent over two years working on ecological projects, the very first thing I should have realised was that you don't disturb the animals. It might have been all right to try and mount a manta ray ten years ago when first I heard about it, but not now. No way. You don't touch the reef. You don't take anything. No shells, no coral. You don't touch the fish, except maybe a few that it's OK to feed. And you certainly don't fuck about trying to ride manta rays.

'Hardly any chance you'd even be able to get near one anyway,' said Ian. 'They're very timid creatures. I guess some people have managed to get to ride on a ray in the past, but I would imagine it would be very difficult. But now, we just can't allow it.'

'No,' I said, rather shame-facedly. 'I understand, believe me. I just hadn't really thought it through, I guess.'

'But we can go and have some fun with the Sub Bug,' said Ian. 'No worries. We can take some pictures too. That's a hell of a camera you've got there.'

We now come to another rather embarrassing part of the story about which I have so far been extremely silent. Some very nice people at Nikon Cameras in England had lent me for this trip a brand new Nikonos AF SR underwater auto-focus camera, which is about £15,000 worth of the most sexy and desirable and fabulous camera equipment in the world. The camera is just wonderful, brilliant technology. Really. You want to take a photograph underwater, this is the perfect thing. It's an astounding bit of kit. Why am I going on about it like this? Well, I spend a lot of time working on a computer, and because I am used to using a Macintosh I hardly even bother to read manuals and so - I didn't really bother to read the manual for this camera.

I realised that when I got the films back.

Please - I really don't want to say any more, except to say thank you very much, Nikon. It really is an awesome camera, and I hope very much that you will let me borrow it again one day. I won't mention the camera again in this article.

We took a small dinghy out to a tiny deserted island about ten minutes away. Ian and I spent a happy hour or so pootling around with the Bug. We dealt with a couple of problems - a grain of sand in a valve and so on. We worked out that the Bug didn't work too well in very shallow water when it had to work against a tide. Well, we'd take it deeper tomorrow. Jane lay in the sun on the beach and read a book. After a while we got back in the dinghy and went back to Hayman. Not much of a story in all that, I suppose, but the reason I mention it is that I remember it very vividly, and one of the shortcomings, I sometimes feel, of somewhere like, for instance, Islington, is the lack of any immediately accessible tiny islands that you can spend the afternoon pootling around with Sub Bugs on. Just a bit of a poignant thought, really. We don't even have any decent bridges you can deface.

There were about ten of us on the dive boat the following morning. The hotel is so spacious and rambling that you don't often get to see many of the other guests, but it was interesting to begin to realise how many of them were Japanese. Not only Japanese, but Japanese who held hands and gazed into each other's eyes a lot. Hayman, we discovered, was a major Japanese honeymoon destination.

The Sub Bug sat up on the back of the boat, and I sat looking at it as we made the hour's journey out to the reef. Hardly any of the Barrier Reef islands are actually on the Reef itself, except for Heron Island. You have to get there by boat. I was very excited. Apart from a couple of refresher dives in pools this was my first proper scuba dive in years. I absolutely love it. I'm one of those people who has been tantalised by the flying dream for years, and scuba diving is the closest thing I know to flying. And for someone who is six foot five and less sylph-like than, to pick a name at random, the Princess of Wales, the sensation of weightlessness is an ecstatic one. Also, I usually vomit on the way back which is a good way of working up an appetite.

We reached the reef, moored, got into our wet suit gear and prepared to dive. At low tide, the reef usually breaks the surface of the water. You can even walk on it, though that is now discouraged because of the damage it can cause. Even when the tide is high, though, reef diving is not a deep diving sport. Most of what there is to see lies in the upper thirty feet and there's rarely any cause to go deeper than sixty feet. The very deepest a sports diver can go is about ninety feet, but there's really not a lot of point. You're usually looking at bare rock rather than coral at that depth, and Boyle's law means that you use up your air much faster down there. Also you have to spend much more time on the surface between dives if you're not going to get the bends. The Sub Bug keeps you at a safe maximum depth of thirty feet.

I wanted to do a regular dive first to get my bearings. Two at a time we clamped our masks and regulators to our faces, did the Big Stride off the diving deck back of the boat and dropped into the water in an eruption of bubbles. A moored dive boat always attracts the attention of a lot of local fish who expect, usually rightly, that they will get fed. The ones you'll get to see if you're lucky are the Maori wrasses, which are extraordinary pale olive green creatures about the size of a Samsonite suitcase. They have large protuberant mouths and very heavy protruberant brows, but the reason for the name Maori wrasse, any Australian will assure you apologetically, is because of some paint-like markings on their brow. Australians are not racists anymore.

There were quite a few wrasses around the boat, and I made the mistake of getting between a couple of them and some pieces of bread that someone had thrown from the boat. The animals blithely barged past me to get at the food.

I sank down to the reef in the huge space of water and light beneath the boat, and drifted easily round it for a while to get used to being underwater again, then came back to the boat to divest myself of my scuba tank and collect the Sub Bug. Together, Ian and I hauled it into the water. I got myself into position behind the thing, and started it up. One of the curious features of scuba diving is that your suit and equipment seem so heavy and cumbersome and unwieldy on the surface - which is one of the things that tends to frighten beginners - but once you descend below the water level everything begins to flow smoothly and easily, and the trick is to exert yourself absolutely as little as possible, in order to conserve oxygen. It is, almost by definition, the least aerobic sport there is. It won't make you fit.

At first I was disappointed that the Sub Bug wouldn't move me faster than I could swim. We were gently pulling our way down, but as I started to get used once more to the slowness with which everything happens underwater I began to relish the long slow balletic curves it let you make through the water, stretched out at full length behind it instead of swimming in the normal position with your arms by your side or on your chest. Following the contours of the reef became like skiing in ultra slow-motion - an almost Zen-like idea. I began to enjoy it a lot, though after fifteen minutes of experimenting with it I began to feel I had exhausted its repertoire and began to look forward to swimming under my own power again. I suspect that it's probably a machine best suited to people who want to experience a dive but don't want to bother with the business of learning to use buoyancy jackets and so on.

I returned to the boat and we hauled the thing back up out of the water. Well, I'd driven the Sub Bug. But over lunch I was worried about the total collapse of my absurd comparative test drive plan and discussed my thoughts with Ian and Jane.

'I think we just have to think about the comparative test drive on a kind of conceptual basis,' I said. 'And we have to award some points. Obviously the Sub Bug wins some points for being portable up to a point. You can take it on a plane which you wouldn't do with a manta ray, or at least, not with a manta ray we liked, and I think that we probably like all manta rays on principle really, don't we? Your manta ray is going to be a lot faster and more manoeuvrable, and you don't need to change its tank every twenty minutes. But the big points that the Sub Bug wins are for the fact that you can actually get on it. I think it has to get a lot of credit for that if you're thinking of it as transport. But then, let's turn the whole thing around again. The reason you can't actually ride a manta ray is a sound ecological one, and on just about every ecological criterion the manta ray wins hands down. In fact any form of transport that you can't actually use would be a major ecological benefit don't you think?'

Ian nodded understandingly.

'Can I get on and read my book now please?' said Jane.

For the afternoon dive Ian said he wanted to take me in a different direction from the boat. I asked him why and he looked non-committal. I followed him down into the water and slowly we flippered our way across to a new part of the reef. When we reached it, the flat top of the reef was about four feet below the surface and the sunlight dappled gently over the extraordinary shapes and colours of the brain coral, the antler coral, the sea ferns and anemones. The stuff you see beneath the water often seems like a wild parody of the stuff you see above it. I remember the thought I had when first I dived on the Barrier Reef years ago, which was this was all the stuff that people used to have on their mantelpieces in the fifties. It took me a while to rid myself of the notion that the reef was a load of kitsch.

I've never learnt the names of a lot of fish. I always swot them up on the boat and forget them a week later. But watching the breathtaking variety of shape and movement keeps me entranced for hours, or would do if the oxygen allowed. If I were not an atheist I think I would have to be a Catholic because if it wasn't the forces of natural selection that designed fish, it must have been an Italian.

I was moving forward slowly in the shallows. A few feet in front of me the reef gradually dipped down into a broad valley. The valley floor was wide and dark and flat. Ian was directing my attention towards it. I didn't know why. There seemed to be just an absence of interesting coral. And then, as I looked, the whole black floor of the valley slowly lifted upwards and started gently to waft its way away from us. As it moved its edges were rippling softly and I could see that underneath it was pure white. I was transfixed by the realisation that what I was looking at was an eight foot wide giant manta ray.

It banked away in a wide sweeping turn in the deeper water. It seemed to be moving breathtakingly slowly and I was desperate to keep up with it. I came down the side of the reef to follow it. Ian motioned me not to alarm the creature but just move slowly. I had quickly realised that its size was deceptive and it was moving much more swiftly than I realised. It banked again round the contour of the reef and I began to see its shape more clearly. It was very roughly diamond shaped. Its tail is not long like a sting ray's. The most extraordinary thing is its head. Where you would expect its head to be it's almost as if something has taken a bite out of it instead. From the two forward points, the outer edges of the 'bite' if you see what I mean, depend two horns, folded downwards. And on each of these horns is a single large black eye.

As it moved, shimmering and undulating its giant wings, folding itself through the water, I felt that I was looking at the single most beautiful and unearthly thing I had ever seen in my life. Some people have described them as looking like living stealth bombers, but it is an evil image to apply to a creature so majestic, fluid and benign.

I followed it as it swam around the outside of the reef. I couldn't follow fast or well, but it was making such wide sweeping turns that I only had to move relatively short distances round the reef to keep it in sight. Twice, even three times it circled round the reef and then at last it disappeared and I thought I had lost it for good. I stopped and looked around. No. It had definitely gone. I was saddened, but exhilarated with wonder at what I had seen. Then I became aware of a shadow moving on the sea floor at the periphery of my vision. I looked up, unprepared for what I then saw.

The manta ray soared over the top of the reef above me, only this time it had two more in its wake behind it. Together the three vast creatures, moving in perfect undulating harmony of line, as if following invisible roller coaster rails, sped off and away till they were lost at last in the darkening distance of the water.

I was very quiet that evening as we packed the Sub Bug back into its big silver box. I thanked Ian for finding the manta rays. I said I understood about not riding them.

'Ah, no worries mate,' he said. 'No worries at all.'