Exposed ... the towering secret under the Millennium Dome

A 60ft tower stands in the way of Year 2000, says Chris Blackhurst

The Millennium Dome, we are promised, will be a showpiece for all that is best about Britain, a monument to this country's proud tradition of design and innovation, a dramatic statement to the rest of the world. Visitors will be treated to a fantastic visual display, detailing Britain's last 1,000 years and looking ahead to the next 1,000 ... and will also be confronted with what looks like a huge chimney.

Details of what the Millennium Exhibition at Greenwich will contain are a deep secret, known only to the teams reporting to Peter Mandelson, the government minister in charge of the project, and Lord Rogers, the architect. But the Independent on Sunday can reveal that the dome will embrace a 60ft-high ventilation tower for the Blackwall Tunnel.

Rather than take down the tower and move it elsewhere, Lord Rogers, famous around the world for putting industrial pipes on the outside of buildings, has decided to include the white Blackwall tower inside the dome. The structure will extend from floor to ceiling and poke out through a special hole in the roof.

Mysteriously, the hole is visible on some, but not all, of the official drawings released by the designers. The best and most frequently used image, released since Mr Mandelson took charge, does not show it.

Tall as it is, the tower alone is not high enough to reach the dome roof. To prevent visitors choking on fumes from the cars and lorries below, a brick extension will be stuck on top.

While people inside the dome will be saved from breathing in traffic pollution, those outside will not be so lucky. Paul Johnston of Exeter University, a scientist for Greenpeace, the environmental pressure group, said there would be a fall-out of potentially hazardous hydrocarbons, carbon monoxide and nitrogen oxide. "There will be a regular old cocktail coming out of that vent," he said.

Dr Johnston said he dreaded to think what might happen if there were an accident in the tunnel involving a hazardous cargo, possibly emitting a cloud of toxic vapour. The organisers, he presumed, would have no choice but to shut the exhibition down.

A spokesman for New Millennium Experience, the government body organising the exhibition, denied that the tower posed any threat to health. "It is a standard ventilation shaft, there is nothing dangerous about it," he said. "Fumes will not be in the dome."

The spokesman said there was no significance in the roof hole appearing on some illustrations but not others. Nobody was air-brushing it out. "They are artistic impressions, not technical drawings. There will be a hole in the fabric where the top of the shaft will just poke out."

Inside the dome, said the spokesman, the vent would not be an eyesore. "It does look huge," he admitted, "but it is impossible to visualise just how huge the dome will be."

That, however, does not solve the problem of what to do with it. Will the tower be covered in murals illustrating British life through the ages? Will children be encouraged to play hide-and-seek around it? Will we all link arms and dance as if it were a giant maypole? When the dome reverts to a sports complex, will people have to play football around it?

Only time will tell.