In 1990, the West Coast millionaire Doug Tompkins landed his small plane in a remote fjord in the Palena region, at the northern limit of Chilean Patagonia. He had visited this ‘quaint tail of the South American continent’ ever since dropping out of high school in the ‘60s and going on to become the millionaire entrepreneur of the Esprit clothing giant. This time however, he wasn’t shooting rapids or scaling peaks. He came to buy, buy, buy.

The ranch he acquired, and later transformed into his home, was the first piece of a multi-million dollar puzzle which today encompasses a thousand square miles of land Darwin described as a green desert: remote, inaccessible and near-pristine. Tompkins owns every fjord, glacier, volcano and tree in an area larger than Snowdonia. From Chile's jagged, piece-meal coastline to the Argentine border to the east, he owns the lot – bar one missing piece in the middle. The bill? A cool fifteen million dollars.

His land harbours some of the last remaining temperate rainforest in the world. Not the sweaty, bug-ridden forest of the Amazon, but primeval, dense-as-history, cold forest, the most species diverse of all. It is also the wettest. "Patagonia of infinite Land and Water, riven by a torrent of love, navigating a single miracle-swollen river," eulogised the Patagonian poet Mario Miranda Soussi. In Patagonia, it rains menageries. Tompkins’s avowed intention is to leave his slice of cold wet jungle just the way it is. Or nearly.

Caleta Gonzalo was once little more than a wharf. It still retains its old status during the summer months, when road users board ferries to continue their journey along the Carretera Austral Augusto Pinochet. The builders of the General’s dream of colonising the south took one look at the rugged, wild land ahead, and decided there was nothing wrong with the ferry. The road only begins again in earnest over one hundred miles to the north, where Tompkins’s reach is finally exhausted.

Caleta is now the showpiece of the millionaire’s Pumalín Park. On the southern shore of Reñihué Fjord, half an hour's boat ride from his house, he has set about building an ecotourist complex replete with restaurant, information centre, organic farm, trails, lodges and camping grounds. Although tourism is by no means the only industry he intends to develop in his dream park, he recognises its potential. He has poured vast sums and much of his seemingly limitless energy and creativity into the project at Caleta.

The result is impressive. Caleta is a Chelsea Flower Show of landscaped gardens, channelled brooks and wooden walkways. Shingled paths lead to swingbridges and hidden campsites, while all around sweeps of mist and cloud swirl over the steep hills. A large panel by the ferry wharf welcomes visitors to the park and explains its objectives. Access to the private lands is granted, it states, on the understanding that none of the natural environment will be altered, damaged or removed. All along the Carretera Austral, completed as recently as 1988, the ashen trunks of dead trees haunt the road like cenotaphs. Colonisers lost control of fires in the 1930s and ‘40s, and for mile upon mile whole hillsides resemble matchsticks scattered by a malevolent giant. In a country where logging, fishing and mining are the chief engines of development, Pumalín sticks out like a green thumb.

Overlooking the fjord, seven delightful, Hansel and Gretel cabañas huddle against the hillside. Their interiors owe much to the en vogue utilitarian Shaker style. Walls painted matt blues, greens and ivories combine with wholemeal-coloured, locally woven bedspreads and cushions. Latticed windows frame picture postcard views, while attention to detail is acute. Carved otters, puma, deer and birds process along the bed bases, floral designs wrap around mirrors and intricate wooden latches replace metal wherever possible. Each cabaña bares the name of a local animal, depicted by the front door and on its key-ring. The cabins are ideal for families, sleeping up to six, with stepladders leading to split-level platforms in the eves. Tompkins was responsible for most of their design, and he and his wife Kris’ tastes are apparent everywhere you look.

Kris busies herself with the arts and crafts kiosk in the restaurant. An information and artesanía centre opens in the summer, but meanwhile there are woolly jumpers to fold, jars of honey to dust and knitted dolls with indifferent smiles to arrange. While refreshing the small vases on the tables with wild flowers, she chats with the Chilean staff in fluent Spanish.

"I take care of the shop and Doug does just about everything else," she tells me, before scurrying back upstairs to fetch more stock. Lively and petite, she comes to Caleta most days to see how work is progressing, and seems imbued with the same creative, effervescent spirit which drives her husband.

Through the criss-cross of exposed beams, copper lampshades illuminate beautiful blond hardwood tables.

"That’s manío wood," she explains. "Every bit is salvaged. We’re not into cutting down forests."

In one corner, a fire blazes under a large copper chimney-piece. When visitors arrive, the hearth becomes the focal point of the room and is transformed into a patchwork of brightly-coloured clothes pegged to every vantage point by their damp owners. You can immediately tell the restaurant was built by North Americans, with their countrymen in mind. In the entrance porch, a discreet notice asks the clientele not to smoke.

Virtually all the food for the restaurant comes from the organic farms nearby. Fresh salmon is caught every day and lamb barbecues arranged for larger groups. The aroma of freshly baked bread pervades the airy space, and its warm, earthy feel comes as a welcome contrast to the near-constant drizzle outside. Large, imposing black and white prints of soaring trees and flowing rivers adorn two walls, as if to remind visitors of Pumalín's mission.

Chile’s Nobel Prize-winning poet, Pablo Neruda, once claimed "Anyone who hasn’t been in the Chilean forest doesn’t know this planet." The gringo eco-warrior’s solution to this state of ignorance is simple. Create a Chilean Yosemite, and encourage people to discover its "fragrant, silent and tangled" forest. Four trails have been cut around Caleta with this in mind.

The most accessible starts from next to the restaurant. After an hour and a half's healthy climb through dense native forest and across one river, the trail enters a gully. Walkways and stepladders guide you along the dripping rock walls until, via a somewhat perilous knotted rope, you emerge at the foot of a forty-metre waterfall. Though enticing, the temptation to dive in is in the region of you must be joking.

Nine miles back up the road which leads to Caleta, three more trails can be walked. The most developed is the Alerce which guides visitors on a circular tour of an hour. Swingbridges ford milky turquoise rivers and crazypaving cross-sections of trunks hop, skip and jump over murky bogs. The alerce is a cousin of the North American sequoia. Both are impossibly old and increasingly rare. Some were saplings when the Sphinx first punctured the desert horizon. At the end of this century however, Pumalín possesses some of the last undisturbed alerce forests in the world.

Woods of alerce, Fitzroya cupressoides (named after Captain Fitzroy of the Beagle), are Nature’s cathedral forests. Their trunks, draped in epiphytes, rise in rectilinear lines, spidery masses of foliage knitting tangled green vaults eighty feet high. Water courses in rivulets down the bark's troughs, while thickets and bamboo buttress the trees in a thick understory. Darwin, who explored the surrounding area in the Beagle, remarked "within the forest, the number of species, and great abundance of mosses, lichens, and small ferns, is quite extraordinary." Two thirds of Patagonian forests’ native plants are found nowhere else, and many are considered rare or endangered. The age of the trees and the biomass of the rainforest make the jungle one of the most successful biomes ever studied. They accumulate as much as two hundred times more organic material than their Amazonian neighbours. Despite this, Chile's temperate forests remain alarmingly poorly protected.

Tompkins is well aware of this. The alerce trail is more akin to an ambulant lecture than a walk in the woods. Didactic wooden noticeboards stand at strategic points ready to inform visitors of the trees' unique qualities and global significance. Handrails ring many of the older trees while others are singled out to show damage inflicted by loggers. The alerce is a protected ‘natural monument’ and its logging banned. However, illegal felling of these Jurassic giants continues. It is thought between 15 and 20 million hectares of Chile's native forests have been lost since colonisation began.

Further up the road lies the Hidden Waterfall trail and another campsite with excellent amenities. In Pumalín, even the watertanks are disguised so as not to offend the eye. There is also a fourth trail, Tronador, the most spectacular and wild of all. At the end of an hour-long walk, a small lake cupped by a natural amphitheatre appears as if by magic, bordered by grass verges perfect for camping. Although all the trails are easily accessible on foot, the regular minibus traffic along the road also provides a less strenuous means of exploring the park.

Christian is one of the young park wardens at Pumalín. He worked for CONAF, the Chilean forestry service, before moving to Caleta early last year. His job is to maintain the trails and inform visitors about rules and regulations. Occasionally, he acts as a guide.

"It's a very exciting project to be involved with," he enthuses. "You feel like you're in at the start of something which will just keep growing. It's also a beacon of hope here in Chile. You look at the destruction all around in Patagonia, even in national parks, and you despair. Here at least is an example of what can be done. With money, of course!"

Tompkins and his park-dreams are still controversial in Chile. Much to his chagrin, he became a bogeyman for the Chilean Right and the military when the extent of his purchases was made public. The missing piece of his giant puzzle, which he had sought to acquire, was sold to, of all people, the state electricity company right in front of his nose. There is little philanthropic tradition in the country, and rumours and fantasies about what a gringo could want with so much land have been rife. No-one can believe he simply wants, in his words, "to put my money where my mouth is."

"That's all calmed down now," claims Christian. "I think people realise his good intentions. As more visitors come to the park, so the message will spread that Doug is sincere and, hopefully, more Chileans will follow his example."

Tompkins has moved to quell opposition to his park by handing over its management to a Chilean-based organisation, the Pumalín Foundation. He is also seeking ‘Nature Sanctuary’ status for his lands, achieving a compromise between the state national parks system and privately owned land. With sustainable management its grail, the project has already begun a programme of incentives and infrastructure to encourage tenants inside the park and its borders to adopt greener production techniques. Organic pesci- and horticulture, honey production and knitted wool handicrafts are all being developed. Scientific research is also high on the foundation’s agenda.

Hundreds of tourists are expected to pass through Caleta Gonzalo. A small hydroelectric plant diverts part of a fast-flowing river providing power for the complex. All the wood used in the construction work comes from reclaimed sources. After initially using landfills inside the park, rubbish is now transported to the town of Chaitén, forty miles away. Problem solved. Doubts about tourism’s sustainability can’t be disposed of so easily however.

Catering for hordes of dayglow-clad daytrippers seems at odds with Tompkins’s philosophy. He is a fervent deep ecologist, advocating a shift to a more bio-centric view of the world – despite owning three planes. Although probably less than 5% of the park will be affected by tourism, mitigating its insidious influence and impact will be Tompkins’s greatest challenge. His philanthropic millions should preclude the Pumalín project falling foul of over-development.

At present only four more tourism nodes are being developed. All of them are small in scale and impact. They will eventually enable visitors to discover more of the park’s wonders, though they remain bound to the coastline and it is unlikely trails will be cut further inland. The rest of the park will remain as it is, a sanctuary from the twentieth century.

In the hardback visitors’ book in the restaurant, well-wishers have filled page upon page with thank yous, words of encouragement and praise for the homely cooking. Some left the park ‘feeling rejuvenated after their time communing with Nature’. Others ‘loved the houses and the food’. There is the odd nationalist "Chile for Chileans" comment. The most striking note however, was penned by a little girl, aged 9. "Thank you for letting me stay," she wrote. "When I have children of my own I’ll bring them to see the forest too."

Pumalín is a long-term project. Its ultimate goal is a change in consciousness on a global level. By allowing visitors into the park, and facilitating their contact with the wilds, Tompkins hopes to contribute to that cultural sea-change. It’s a tall order, but one which Tompkins obviously has no fears of taking.

"If you would see his monument, look around," reads the inscription above the North Door of St. Paul’s Cathedral. When a little nine-year old returns as a mother, it would be fitting if, deep in the tangled undergrowth of one of Pumalin’s cathedral forests, a new panel with the same inscription had appeared. On salvaged wood, of course.