We skid to a halt in a clearing, and a cloud of dust fills the air. As it dissipates, the landscape around us is revealed: the lush green trees first, then the carpet of sasa bamboo, followed by mountain slopes stretching back up to snow-capped summits. As I suck in the clear mountain air and lean on the handlebars, one feature draws my eye, demanding attention: across the valley towers Mount Yotei, golden in the evening's fading light.
No matter how many times I cast my eyes over its 1,898-metre peak, I never cease to be amazed. This long-extinct volcano dominates every vista, the violent nature of the region shaping the land and producing steaming hot spring baths, open beneath the clear, starry skies. Mountain biking may have become a universal sport, but this landscape is unique. There is only one place on Earth I could be – and it's a place that averages an earthquake a week. Hokkaido is an active place, in every sense.
Here on Japan's northernmost island, winter sees a soft duvet of snow lying across the region, luring skiers from the rest of the country. In summer, however, a jagged, Jurassic landscape is draped in an almost tropical-looking carpet of green, fringed by the crystal blue of the Sea of Japan to the west and the roaring Pacific to the east. The sun, tempered by cooling winds, draws visitors from a mainland sweltering under an oppressive blanket of humidity.
Nearby, the bustling metropolis of Sapporo, the country's fifth-largest city, provides that dazzling sensory collage that satisfies one's preconceptions of urban Japan. Traffic, shopping, crowds and street dancing are all set against a background noise of Pachinko halls and banks of neon billboards that sing out competing advertising slogans. But within half an hour of its centre, you can be among wide valleys, where shrines wait in shady woodlands and herons stalk shimmering paddy fields.
"Here's a picture of me with the police," laughs Noboru Tagawa, handing me a faded photo. "They are writing me a ticket for surfing. This was 1979." Noboru was Hokkaido's first surfer and now runs a ski school at one of the island's winter resorts. "They thought surfing was dangerous and irresponsible. Whenever we went in, they would come with loud hailers and call us from the water. So we set up a surfing federation, and proved to everyone surfing was a great thing to be involved with."
Adventure sports are new on Hokkaido, but surfing helped to lead the way. In ski and snowboard resorts, rafting, kayaking, climbing and mountain biking are taking off in the summer season. The ski lifts may be silent, but the mountain trails and rivers are drawing a new generation of thrill-seekers from across Japan. Attitudes have changed: there is now a deep appreciation of the natural environment and the ways it can be enjoyed.
There's something exhilarating about white-water rafting on Hokkaido. In sections, the river seems to slow as if drawing breath, as you paddle into position for the next set of pulse-quickening rapids. Then: a weird feeling as the front of the raft drops over the edge into the boiling white water, a moment where it seems like the back of the boat is struggling to catch up, and suddenly the acceleration kicks in – like a roller-coaster dropping over a precipice.
But it's not all about bulldozing along with your eyes shut on a white-knuckle ride. It's more a way to interact with the landscape, going into the next thrilling encounter with your eyes wide open.
"We have good spring waters, great big waves," says Ross Findlay of the Niseko Adventure Centre. "For those that want to have an explosive time, come in the spring. Then when summer comes, everything tames down and you can just float down the river, a couple of waves here and there, and lots of pools to swim in."
Ross came to Hirafu for the world-class skiing. Nearly two decades later, he's still here. "There wasn't anything to do in summer, but I enjoyed kayaking on the river, so we decided to start a rafting business. We had a one-boat operation which we ran from a car park. It really took off. In our third year, we had 15,000 people. We were offered this school gymnasium as a base, so we broke it down by hand, moved it to this site and rebuilt it." To Ross, it was about showing the local community he meant business.
The Niseko Adventure Centre is housed in a striking two-storey wooden structure with a distinctly organic feel, in tune with its surroundings. The huge glass frontage frames Mount Yotei, drawing the eye of those enjoying a coffee in the upstairs café. "We're now in our 15th year, and offer sea kayaking, mountain biking, trekking, canyoning, climbing and have a trail-run through the mountain."
Niseko still has some way to go growing its summer options beyond rafting – but that's part of the charm. This isn't a polished outdoor product yet – you're getting off the beaten track.
Ross has been pushing the development of a mountain-biking infrastructure with the authorities following the success of the sport at resorts in Europe, especially in Scotland. "There are tracks here, but I've got them to consider putting a trail around this mountain, like a loop course, which would be a really fantastic ride. Most of the roads are already in there, it's just a case of connecting it all up."
At the foot of Annupuri, the village of Hirafu is one of the world's leading snow resorts. If you want the very best in free-riding and off-piste powder, Annupuri can supply it. In the past 10 years, this part of Niseko has exploded in popularity, with luxury new builds springing up to accommodate those dropping in for a ski fix.
In spring, the village is laid bare, the clean modern lines of the buildings seeming to fit with the natural environment. It's all angles, cedar cladding and huge, glass-fronted balconies. One downside is the bars and restaurants that create a buzz in the winter melt away in the summer – downtime means relaxing aching limbs and soaking in one of the onsen (hot springs).
In Hokkaido, the experience of the great outdoors comes together like a sushi roll – where layers, textures and flavours of Japan are woven together, with a wasabi-like twist of adventure thrown in, combining to make more than the sum of its parts. The town of Niseko is shaped by the mountains surrounding it – they aren't just a backdrop, but a resource and an adventure playground. Your taste of the island will depend upon when you visit. But whenever you come, you will find an active landscape waiting.
The writer travelled with Japan Airlines (0845 747700; uk.jal.com;), which flies twice a day from London Heathrow to Tokyo and has connections to Sapporo in Hokkaido.
Hokkaido Tracks (0081 136 233503; htholidays.com) offers modern luxury apartments and houses to rent year-round within the village of Hirafu, with rates of around £500 per two-bedroom property per week.
Niseko Adventure Centre (0081 136 232093; nac-web.com) offers rafting for Y6,000 (£40) per half day, half-day mountain biking tours from Y6,000, including bike hire, and a day of rock climbing from Y9,800.
Scott Adventure Sport (0081 136 213333; sas-net.com) offers white-water rafting for Y5,250 per half day and canyoning for Y6,300 per half day.
Niseko Outdoor Adventure Sports Club (noasc.com; 0081 136 231688) offers a range of summer activities including rafting, mountain biking, mountain boarding and abseiling. A half-day of mountain boarding or mountain biking costs Y4,725.
Japan National Tourist Office: 020-7398 5678; seejapan.co.uk