A 60-metre deep crater near the Cornish village of St Austell is being turned into a garden the size of 30 football pitches. “This place is going to be here for ever, so when you’re old and wrinkly you’ll be able to say ‘I saw the Eden Project when it was being built’,” education co-ordinator Jo Readman tells a group of Year 9 pupils from Exeter.

They are in a portable classroom perched by the old Bodelva china clay pit, which was worked for more than a century, creating a 14-hectare bowl. At the moment it is still a building site.

Colossal geodesic dome conservatories, called biomes, designed by Nicholas Grimshaw and Partners, already squat against the wall of the crater.

The largest, the Humid Tropics Biome, would cover the Tower of London. By next spring, it will have become a “rainforest cathedral”, a lush jungle housing plants from Amazonia, West Africa, Malaysia and the Oceanic islands. It is the biggest conservatory in the world.

Teak, balsa and mahogany trees, already waiting in a nearby nursery, will grow almost up to the roof. Paths will wind up and around the steep side of the forest, taking visitors past coconut palms, mangrove pools, a lagoon and up to a 25-metre waterfall.

An area of “slash and burn” will show how people clear land for crops including cassava and palms. Trails leading from the forest will feature wild plants used for food, tools and medicine. Another section will show farmed plants such as rubber, cocoa, vanilla, bananas, rice and coffee.

The Warm Temperate Biome will host plants which like a Mediterranean climate. Agriculture will be contrasted with the natural flora of southern Africa and California.

Outside, a series of interlocking crescent-shaped terraces will contain more familiar plants from the temperate zone, including Chile, China, Japan, the United States, New Zealand and Britain.

One of the reasons Cornwall was chosen from this epic venture was its mild climate, clean air and ample water, all vital for such ambitious cultivation.

Above the terraces, plantings on the steep sides of the owl will evoke the wild places of the temperate regions. At the heart of the crater, there will be a lake, surrounded by displays celebrating the plants of myth and folklore.

Eden will then close its gates until the spring, when the buildings will be ready and the 80,000 plants in place.

“Will there be any animals?” asks one pupil . “No mammals,” replies Ms Readman, “but there’ll be lots of insects and maybe lizards and reptiles to eat the pests, because we want to be as organic as possible.”

A school visit includes a journey in an open-sided train to near the bottom of the crater for a close-up look at the biomes, plus an introductory session in the classroom and time studying the imaginative displays in the visitor centre at the top of the crater.

Next year, the education resource centre will be open in the Biome Link, a long, low building joining the Humid Tropics and the Warm Temperate biomes.

Eden’s declared mission is to make people more aware of how much we depend on plants. It’s determined to do this in a dramatic and entertaining way as possible. The Exhibition Gallery is full of Heath Robinson-style mechanical wizardry that demonstrates this vividly. For example, there is a family kitchen tableau which, with a whirr of pulleys, turntables and pneumatics, gradually empties of everything that has come from plants, from the tables and chairs, to clothes, books and food.

Next year, schoolchildren will be offered total immersion in the Eden experience, with “expeditions” to different parts of the world, detective workshops to trace items of clothing back to its country of origin, cooking workshops to watch food being prepared and find out where it came from, plus drama, role playing and scientific experiments.

“Our educational policy is more Harry Potter than Pokemon,” says Ms Readman. “It’s our aim to make plants exciting; there’s no dumbing down. Whether people are three or 93, we treat them as if they are intelligent.”

“It’s an idealistic project and it shows,” says John Spivey, a physics teacher from Exeter. “I’m very impressed by the construction and the message and I think the kids are too.”

The project will cost pound;80 million, including pound;37.5 million from the Millennium Commission.