Since when, you might ask, have architects dreamed up shows as well as buildings? Since Inigo Jones in Jacobean times, as it happens, but that's another story. Fisher trained as an architect in the 1960s, but quickly got into designing stage shows - especially for rock bands - and now does little else. The Rolling Stones always employ Fisher. So, usually, does U2. He has designed for Prince, for Michael and Janet Jackson, Tina Turner, for REM, for the Pink Floyd. It's architecture, all right - the sets are like kinetic cityscapes - but what he has in mind for the Dome is rather different.

At the centre of the Dome will be a very large circular arena, with a raised audience promenade around it. It is 150 metres across and 50 metres to the roof. Fisher says that the oft-quoted comparison with Trafalgar Square is apt - not only is the Dome as high as Nelson's Column at its apex, but the central arena alone would touch the edges of the Square if they were superimposed. You have to imagine you are on the steps of the National Gallery, watching people down by the Landseer lions: that's the scale of this "theatre". Fisher likes the emptiness of it, the way it reveals the insubstantial vastness of the Dome's construction. "The thing about it is that it floats. The air inside it weighs more than it