Architecture: Welcome to the new Kew Gardens

In its vast crater in Cornwall the Eden Project site is ready for Nicholas Grimshaw's hi-tech greenhouses.

Pink Floyd playing Dark Side of the Moon at a concert in Cornwall during the eclipse? It seemed like a great idea to Tim Smit, the entrepreneur who discovered the Lost Gardens of Heligan and made them Britain's most-visited private garden, but the Cornwall Council rejected it. Fortunately, his other great idea to put Cornwall on the map, called the Eden Project, did get their go-ahead two years ago.

The Gardens of Eden will open at Easter 2001 in a disused clay pit in north Cornwall. They intend to repair the damage that man has done to the landscape by opening gardens with three climatic zones as an environmental tourist attraction inside a disused open mine.

Two hundred thousand tons of kaolin, the chalky clay that whitens paper and makes porcelain china, is exported from the county every year. The quarrying has turned bits of north Cornwall into Clanger country with craters 60 metres deep; their ziggurat terraces look like the Aztec ruins. When an iron oxide seam which stains the clay red closed a pit the size of 15 football pitches outside St Austell, the Eden Project was born.

The architect, Nicholas Grimshaw, sees it as bringing together two traditional British creative concerns, "building wide-span structures on the scale of Brunel's rail stations and building greenhouses on the lines of Paxton's Crystal Palace. An interest in plants and an interest in structures". He is reinventing the greenhouse in modern materials and making it more beautiful and fit for its purpose than its cumbersome predecessors. Glass greenhouses are dinosaurs today.

"Since this project began, my own feelings about plants have changed fundamentally," Grimshaw admits. "I've become enthralled by the range of sensory perception that the architecture could engender."

Inside his giant geodesic domes, called biomes, there will be year-round shirtsleeves weather replicating the humid tropics and temperate zones. Why travel to South America when for a pounds 9 ticket, you will be able to visit the steamy Amazon in Cornwall? Ferns will cascade from rocky outcrops, lianas and orchids will grow on tall hardwoods and Victoria Regis lilypads as big as paving stones will be floating on ponds. All these specimens are already growing under computerised showers in nearby nurseries and will be transplanted next summer when the biomes are completed.

Nicholas Grimshaw's great crescents of greenhouses will land as lightly as soap bubbles on this fragile moonscape. Where different biomes meet, they fuse into one another with a steel arch, sometimes as tall as 30 metres. They will be constructed without a single pane of glass using a honeycomb of transparent Teflon foil, strong enough to take a man's weight. Three layers of this foil, each as thin as clingfilm, are sandwiched with air and inflated into the pencil-thin, hexagonal steel frames, sometimes 10 metres in diameter. Glass would have been far too heavy and would have required far heavier steel frames, which in turn would have reduced the light.

Nothing prepares the visitor for the colossal scale of the project - what its marketing manager Paul Travers calls "the wow factor". The pit will hide it until the visitor is right at the edge, looking down from the viewing platform. Not even the tip of the tallest biome, which will be 50 metres high and taller than Nelson's column, will be glimpsed till then.

Environmentalists, landscape gardeners, botanists, architects and engineers have had to move millions of tons of earth to build Eden. Geologically the site is granite, but rotten granite with decomposed kaolin, as slippery, shiny and soft as face powder. It's like building the hanging gardens of Babylon on a stack of porcelain plates. English Clay Ceramic Industries have sprayed the sides with water full of grass seed and clover to hold the banks while willows take root.

Chris Smith, who holidays in Cornwall and knows Tim Smit's Lost Gardens of Heligan, is chairman of the Millennium Commission. He approved of Eden Project's transformation of "a lunar landscape into what is going to be one of the major environmental projects in the world". The Millennium Commission promised pounds 37m lottery money on a total budget of pounds 74m. Not as much as the Eden Project wanted - initially, they bid for half of pounds 106m - but Travel and Leisure advisors on lottery handouts calculated their plan on a projected 750,000 visitors annually.

"It's silly when Cornwall gets 4.4 million tourists annually to only expect 18 per cent of the market," Eden's marketing manager Paul Travers believes. "What we learnt from Heligan Gardens is not to underestimate your success."

But the former chief executive of the Eden project, Evelyn Thurlby, who left in June, is convinced that 750,000 visitors is a realistic figure. There were several hurdles to face during her time there. She had to deal with the construction price inflating 30 per cent over budget, savings having to be made in days and then the bank pulling out without notice. But she is adamant that she left Eden in good shape with its funding package completed, construction well underway and on schedule for its opening.

"The teamwork on site at Eden was terrific. Without Tim Smit it wouldn't have happened and Grimshaws were the most responsive, least prissy architects, making sure that Eden functions properly."

As a consequence of her business plan, elements of Eden have been downsized. The desert zone has been cancelled, and plans to develop a hotel on a nearby 60-acre site have been scrapped. The visitor centre has also been scaled down.

It is ironic that once the local population had understood that this lottery money could not be used to fund schools or hospitals, which are Treasury responsibilities, the remaining protests about this environmental project were all green ones: there was a lot of concern about all those cars for the 2,000 parking places and the potential for road rage and traffic jams in the narrow lanes of Cornwall. "As if all 750,000 visitors, each one in a car, will come on the same day," said an incredulous Paul Travers.

"We predict that on a Bank Holiday Monday in Cornwall we will get one third of the number of cars you get at the local Asda on a Friday in winter." But they have designed a road infrastructure to carry cars on dual lanes and disguised car parks as much as possible behind banks of plants on different levels, like rice paddy fields.

"If this lost valley in Cornwall attracts visitors from all over the world, and if they can learn something about the dynamic and changing world of architecture while they learn about the mysteries of the botanical world, then I believe we'll have achieved something," says Tim Smit, now acting chief executive of the Eden Project, while heads are being hunted.

More than just tropical, Eden is a topical project too. Great projects on a similar scale that were built at the end of the 19th century demonstrated man's superiority over nature. In a hundred years, man seems to be learning to be subordinate to the earth and to put back what he has plundered.