Just one hour from Tokyo, the Izu island chain is a world of dramatic volcanic scenery, superb wildlife, hospitality and hot springs. Carl Thompson soaks it up

Here's a question: which capital city contains within its official borders virgin forests, lava domes and eerie deserts of volcanic rock sloping down into one of the world's most pristine marine environments? If you correctly identified Tokyo, you must be one of the few people familiar with Japan's best-kept secret, the seven Izu islands.

As the midsummer humidity presses down in the concrete canyons of the airless capital, the prospect of sea breezes and cool greenery is irresistible, especially with an unspoilt subtropical archipelago just 45 minutes away. In my haste to board one of ANA's four daily flights, however, I forget about Japan's short but intense rainy season and neglect to check the weather forecast, which would have alerted me to the virulent monsoon system parked squarely over Hachijojima, the first stop on my week-long itinerary. It takes skilful manoeuvring by our pilot to squeeze the brightly painted Super Dolphin under the shelf of black cloud and skim onto the runway. Any visions of a sunny sojourn dissolve in the driving rain that chases us into the tiny arrivals hall.

Emerging from the other side with Mr Maeno, my unfailingly courteous guide for the week, we thread our way by taxi into a valley formed by two hulking stratovolcanoes, home to the island's 9,000 residents. A bushy-tailed mongoose scuttles across the road and up into the dense woods, where tiny green fluorescent mushrooms glow alongside a giant chincapin tree measuring 30 metres in circumference. The air rings with alien-sounding birdsong.

The fact that we're still officially in Tokyo already seems purely notional. Indeed, each of the islands in the group nurtures a distinct identity, often expressed through handed-down crafts and forms of dress. We visit a shochu distillery (a kind of sake made from barley) and a workshop producing kihachijo silk. A kimono on display looks like it might make a nice present for someone, but after noting the £45,000 price tag, I decide to look elsewhere for souvenirs.

Outside, with the sky darkening, we abandon hope of getting dry and sensibly head to a rotenburo, an outdoor thermal pool. From the cliff-edge vantage point of the Miharashi-No-Yu spring, we gaze down at the black lava plateau lining the coast and the churning Pacific beyond, oblivious to the storm around our heads that snatches away the clouds of steam.

As evening approaches, we begin to wonder what people on the islands do for entertainment: there are no cinemas or theatres, most islands have no restaurants, and "cow sumo" bouts are a thing of the past. The only pachinko (pinball) parlour is too far away, and the manicured streets are damp, dark and deserted.

Then we see a solitary light coming from a tiny, box-like building beneath a sign saying "Anchor Pub". We fall through the door and topple through a hole in the fabric of the universe. At least, that's how it feels: on the other side there's light, warmth, indie music on the stereo, meat pies in the microwave and an Australian barman welcoming us with pints of English ale on tap. Thus begins one of the most surreal evenings I can remember, during which I find myself continually glancing outside to confirm that this really is a Japanese island in the Pacific.

Scott and his wife Noriko fill in the local picture. They've no regrets about swapping the stresses of Tokyo for the languid pace of island life. However, they're now coping with a different set of climate-related problems. "The typhoons in winter can stop the shipping routes, cutting off the lifeblood of the islands. That means bread soon disappears from the shelves and people start panic-buying," explains Scott.

Tidal waves are another worry, with properties above the supposedly safe 30m mark commanding higher prices - not that the ravenous termites make any such distinction. Then there's an outside chance of sudden eruptions, like the one that forced the temporary evacuation of neighbouring Miyakejima a few years ago. Scott and Noriko stress that islanders make their own fun: hence their decision to establish this fabulous microcosm of Anglo-Aussie culture.

Accommodation in the islands is dominated by family-run minshuku, which means tatami mats, futons, shared baths and tiny towels. Most of all, though, it means fulsome, lovingly presented evening meals comprising up to 15 dishes. Night after night, we pick our way through thick tangles of soba noodles, home-made tofu, grilled and sashimi-style flying fish, sizeable shiitake mushrooms, pungent horse mackerel and gallons of miso soup, knowing that to leave even a mouthful is to risk offending the cook. (In fact, having too much of a good thing is the only complaint I can muster by the end of the week.)

Next morning, the rain has eased, but the wind speed of 16 knots is still enough to ground the helicopter we'd planned to take to the next island. Instead, we charm our way into a first-class berth on the Sarubia Maru, a classy 5,000-ton ferry with Häagen-Dazs dispensers on every * * deck. Despite the soft carpets and net curtains of our spacious cabin, our mood is anxious; partly because of our Hobgoblin hangovers, but mainly because the rising tide looks like preventing us docking, thereby submerging our whole itinerary. It's a close call but we make it, and step thankfully down onto the jetty at Mikurajima.

If the 70sq km of Hachijojima had felt sleepy, our perspective is now radically altered. Mikurajima has a real frontier feel, with just 260 people clinging to the steep northern slopes of an island that epitomises primeval beauty. Deep virgin forests, breached only by several million streaked shearwater (petrel-like seabirds) and a populace of wildcats, blanket a foggy volcanic cone less than two miles across; numerous spring waterfalls pour out around the rocky coastline; the fins of 160 bottle-nosed dolphins dip in and out of the choppy waves. They, in fact, are the main attraction for the 10,000 eco-tourists that come here every year.

One of my companions on the popular dolphin-watching excursion early next morning is Justin, an American research student and Mikurajima's only gaijin resident. As we pitch queasily over the swell, he tells me about echo-location, the high-pitched bio-sonar squeaks with which dolphins locate and size up objects in their vicinity. Before he can finish, a group of perhaps 10 adults surface nearby. Masks on, snorkels in and over the side.

One good way to wake up, I find, is to plunge into the sea as half a dozen three metre-long marine mammals race up at you, bouncing ultrasonic clicks off your internal organs to decide whether you might be worth eating. They circle me once, well within touching distance, before concluding I'm neither a fish nor much of a swimming partner, and glide back into the blue with a swish of their muscular tails. It only lasts a few seconds, but it's a mesmerising experience and we repeat it three times.

Suddenly the pace of the trip has picked up. "Thompson-san, you look like James Bond in that wet suit," declares my guide back at the quay. And that's just how I feel as I throw off my flippers, jump out of the boat and we rush to catch a helicopter to Oshima, the largest island in the group.

An hour later we're 80km north, standing on the rim of the cavernous crater caused by the eruption of Mount Mihara in 1986. Ribbons of mist linger over a haunting landscape of twisted lava interspersed with luminous green knotweed. It's not even midday.

The following day, the sun finally comes out. As Maeno-san heads back to Tokyo, I move on to Niijima on the Jetfoil service, which skates over the waves at a breathless 45 knots. We bank around the pastry-cutter coastline of Shikinejima, where the sea literally boils with seismic activity, and dock minutes later at the venue of numerous championship surfing events.

After checking in at the latest minshuku, I waste no time in borrowing a bicycle and making tracks for the six-kilometre, arrow-straight Habushiura beach. On a hot and clear Saturday afternoon like this, it's bound to be packed. When I arrive, I get the biggest surprise of the whole trip: apart from one black-tailed gull, it's deserted. Do the islanders know something I don't? I flop into the milky blue breakers, half expecting the fatal embrace of a box jellyfish, and find only the welcoming waters and majestic mountain scenery that I've come to expect from the Izu islands. Never before was Tokyo this much fun.