By Julian Joyce
BBC News, Heathrow
As it looms out of the morning mist Terminal 5 doesn't look that big - the first impression is of a long low shed with a gently curving roof.
But that impression is deceptive. It is only when you get up close and enter this vast space that the sheer size of the building makes itself apparent.
Everything about the £4.3bn project is epic, and the height - the equivalent of a 15-storey residential building - only fails to make an impact because the rest of the structure is so vast.
The floor area of Terminal 5 over its five floors is equivalent to 50 football pitches, and the whole site including car parks and ancillary buildings is as large as London's Hyde Park.
The project took five and a half years to complete - following Britain's longest planning inquiry which lasted four years - and employed a total of 60,000 workers.
More than 80,000 tonnes of steel were used - 17,000 in the roof alone. There are 5,000 doors, 800 toilets, 20,000 power sockets and 1,700 miles of cable.
But according to Andrew Wolstenholme, director of capital projects for the airport's operator BAA, and formerly director of construction for Terminal 5, the building being officially opened only represents a fraction of the total work completed.
"It's not just a big building," he says. "What people don't realise is that there were 16 separate projects that we had to run simultaneously to make this whole thing work."
For example, in order to get passengers to the new terminal, the Heathrow Express line needed to be extended, and a link built to the nearby M25 motorway. By the runway, a brand new air traffic control tower needed to be built.
The total project - which is estimated will see about 30 million passengers a year passing through its doors - is the equivalent to completing a second Gatwick Airport, says Mr Wolstenholme.
Leaving aside the controversy of whether the UK needs extra airport capacity, there was a feeling at Friday's opening that Terminal 5 represented a rare British success story - a huge capital project that was completed on time and within budget.
This pride was apparent among the four workers from Palmers Scaffolding who, in their hard hats and reflective jackets, had come to mix with the besuited journalists and VIPs at the official opening.
One of them, Mark Smith, said: "When we started, they called us all together and told us that we would be proud when we had finished building this.
"At the time we all smiled sympathetically. But, now we are here, we are proud."
As part of the scaffolding team, the four men were responsible for making every part of the huge structure accessible to all the other workers.
"All the little places up there," said Doug Carnihan, pointing to the vast roof held up with huge steel spans." That's where we've been."
Architect Mike Davies aimed to give the terminal a "piazza" feel
Scaffolders played a huge part during the construction of the huge terminal which, because of its position, brought its own unique challenges.
Because of low flying aircraft, normal building cranes that would have lifted the roof into place were banned and everything needed to be lifted into position from the bottom up.
Huge 2,200-tonne sections of the roof were winched slowly into place up cables using stand-jacks.
Terminal 5 architect Mike Davies likens the process to a "monkey climbing up a rope".
He and architectural partner Richard Rogers designed the terminal with sheer space in mind.
Mr Davies said the aim was to recreate a "piazza" feel - what he described as a "piece of city under a roof".
He wasn't surprised that the project had been brought in on time and not over budget.
"It's been pretty heavily managed by BAA," he said.
BAA's Andrew Wolstenholme gave an insight into how this was achieved: "Before we started we looked at about 12 other big projects and we found it wasn't unusual for them to come in more than a year late and over £1bn over budget.
The mistakes in the past have, he said, stemmed from builders' inability to manage change - because things change all the time - the relationship between the builders and the clients, and the inability to manage risk.
"What we designed was a new way of managing the project. We [BBA] took all the risk on board and in return for that we asked all of our suppliers to put their very best people on the job," he said.
"By assembling all those people into the right teams we got them to manage that risk on our behalf."
That way, he says, BAA managed to avoid the scenario, familiar to many observers of British public projects, where lawyers for the various project stakeholders have been employed to deflect blame away from their individual clients.