LONDON // By the time the Olympic and Paralympic Games end in London, millions of visitors will have crossed the vast bridge at the entrance of the Olympic Park.
They will have been struck by the sheer scale of the stadium and the swoop of the Aquatics Centre's winged roof. They will have stood, puzzled, by the towering Orbit and been awed by the alien-looking Velodrome cycling venue. They will have wandered by the riverbank and felt, suddenly, as if they were in another world - one of wild flowers, nesting birds and wetlands. Then they will have returned to the throng of the Games.
They probably would not have known that the site is one of the greatest engineering feats in modern times.
In less than three years, this vast site, which takes in four of London's most deprived boroughs - Newham, Tower Hamlets, Hackney and Waltham Forest - was transformed from a wasteland into the largest park established since Victorian times.
The soil and groundwater had been contaminated by more than 150 years of industrial waste. Discarded refrigerators were piled high in the midst of railways sidings, concrete plants, bus garages, chemical works, plastic and glue factories, an oil refinery and a tar distillery.
Today, even Mike McNicholas, the man who led the project to clear, cleanse and reclaim the land, admits that there were moments he didn't think it possible.
But Mr McNicholas, the project director for Atkins' Olympic Delivery Authority (ODA), is used to building the impossible. He did it 10 years ago in Dubai when he led the team of design engineers preparing the ground for the Burj Al Arab - known in the trade as "Mike's island".
"I struggle after 30 years with Atkins to think of any projects on the scale of the Games," said the managing director of Atkins' Design and Engineering division in the UK.
"The 2012 Games had sustainability targets set to make it the greenest games ever, meaning we had to find ways to recycle, reuse and reduce waste on a scale never seen before this type of project."
"The complexity," he added, "was unprecedented. But once we cracked the stadium I started to believe anything was possible."
Mr McNicholas said the London 2012 project has been watched with great interest by Middle East investors.
"In the Middle East, whether it's the UAE or Qatar or Kuwait, speed of delivery is very important. A lot of projects are about legacy - social, political or personal - and achieving that within a short period of time is key."
To that end, the success of Mr McNicholas's work is, literally, that on which all else rests.
Mr McNicholas and his team - ultimately more than 1,000 worked on clearing the site - began their task in September 2005. In April and May 2008, they handed over the stadium and Aquatics centre sites three months early.
They created a "soil hospital" to treat the contaminated ground and reuse it: 30,000 square metres of earth were bio-remediated (meaning naturally occurring bacteria broke down the chemicals into harmless byproducts), 50,000 square metres were chemically stabilised and 700,000 square metres were washed.
Conventionally reusing 50 per cent of the existing soil would be considered a success. The sustainability targets meant that Atkins had to reuse 80 per cent.
The equivalent of 10 football fields worth of Japanese knotweed - one of the most invasive breeds of plant - was cleared. During the process, 140 archaeological trenches were dug uncovering a 19th-century boat, an 18th-century road, skeletons from the Iron Age and a hut from the Bronze Age.
Part of the remit was to revitalise the wetlands and river edge that covered the 8 kilometres of riverbanks that loop around the north and south side of the site. Had the engineers found, for example, nesting water voles, the entire project would have stopped and a multimillion pound rethink would have been required.
"They're protected species," Mr McNicholas explained.
As it was, thousands of indigenous species of newt and bird were carefully removed during construction then returned to their habitat once work was done.
All overhead power lines had to be put underground to provide a power supply resilient enough to fuel the Games. More than 100km of trenches were dug to take cables running through 140 substations. On any given day, dozens of Atkins' engineers can be found at the Olympic Park checking that all is as it should be.
"Just imagine," said Mr McNicholas. "The starting pistol goes on the 100m final and suddenly the power cuts. Who would want to be the man telephoning David Cameron to explain why nobody saw the end of the race?"
The notion of legacy has been built into the fabric of the park and its design. When the Games are over, each of the four boroughs will have its own "legacy venue". But, as Mr McNicholas points out, "there's not much point in having that if nobody can get to it."
In all, 23 bridges connect the park to the surrounding area: seven motorway bridges, three land bridges and 13 footbridges. Ten of these are currently bulked out by temporary decking making them broad enough to accommodate Games' visitors.
In the months after the Games, more than €100 million (Dh572m) will be spent transforming the park again. The temporary venues will be removed and others amended - the wings will come off the Aquatics centre and the screens and lights above the stadium will be dismantled. The bridges, over which millions walked to see the top athletes in the world compete, will be stripped back to leave more attractive permanent links to this particular legacy of the Games.