Smit is a publicity wizard and the giant Eden bubbles, with ant-like construction workers swarming round them, are a genuinely awe-inspiring sight. Even so, I was frankly amazed at the size of the crowds thronging the place when I paid Smit a visit, mid-week. It comes as no surprise to him. "People need things to uplift the spirit," he says. "that's why they go to the Guggenheim in Bilbao. I've spoken to lots of visitors here, and one of the common threads in what they say is that they love the idea of people doing something that's incredible - that's bigger than the sum of its parts. People say, thank you for doing something that's big. Big in every sense - not just big in terms of its architecture, big in terms of ambition."

This is true. As we speak, sitting in the main concourse, visitor after visitor spots Smit's TV-familiar face and comes up to shake his hand, ask him to sign a souvenir book, or whatever. I examine these people closely. They can't all be from Central Casting. No, but they're a type: mostly retired Middle England, time and money on their hands, determined to have a good day out, receptive to spectacle. But Smit thinks he can widen his audience beyond this economic bedrock, particularly in winter when, he says, his great "biomes" will be at their most spectacular. "We're assuming that people who like gardens will come here - but the reason we've set ourselves up as an institution is to make connections with people who don't think they like gardens."

Moreover, he says, he wants the experience - all to do with mankind's utter dependence on the plant kingdom - to be uplifting. Rather than merely tell us how we are destroying the world, Eden will offer hope. Smit hates worthiness with a vengeance. "I do not want to go somewhere to be made to feel guilty," he declares, puffing on his trademark thin cigar. "I don't mind someone pointing out to me that there are certain things wrong with the world, but I want to have the balance of being told: here's what you can do about it. You've got to be optimistic in your delivery."

In all this, Grimshaw is key. He is the quiet optimist to Smit's demonstrative version. Unlike his slightly older contemporaries Richard Rogers and Norman Foster - both now Lords - Grimshaw has never been a public figure, nor had an exotic private life. Significantly, the average age of the 120-strong Grimshaw office is around 30. This goes a long way to explain why - after more than three decades of designing - his studio is now producing some of the most innovative architecture in Britain, Germany, Spain, Holland, Switzerland and - following more competition wins - America.

Grimshaw and his partners, says Smit, have their heads screwed on and aren't precious: when he objected to an earlier design for a visitor centre, they came down to see him with pencil and paper, set to it, and came up with something totally different and much better. Grimshaw married modern and ancient technologies for this "Gateway to Eden" as Smit calls it: a high-tech column-free roof floats above walls made of rammed earth. He and Grimshaw are not only planning to expand Eden, but are looking at ways to make new, Utopian communities in other industrial wastelands in the area. "Nick's up for it," says Smit with admiration.