Six flights of stairs below a muddy construction yard next to the Billingsgate fish market in East London, a gang of 20 miners, their faces smeared with mud, are darting along metal platforms wedged between the sides of a cavernous tunnel and a giant cylindrical machine that stretches as far as the eye can see.
The atmosphere is muggy and there's a thick smell of clay, as a giant archimedes screw spews out earth dug from the tunnel face a few metres away.
These miners are operating a 145m long, 1000 tonne beast nicknamed "Elizabeth", which is slowly digging its way from Canary Wharf to Stepney Green by the end of the year and then on to Whitechapel and Liverpool Street. Its final destination is Farringdon by the end of next year.
From the surface of Canary Wharf, where office workers scurry between the glass-clad headquarters of some of the world's biggest banks, there are no clues that beneath their feet, miners are working 24 hours a day, seven days a week on Europe's biggest infrastructure project.
These men are working on Crossrail, London's newest railway line, which will connect Maidenhead and Heathrow to the west of the capital to Shenfield and Abbey Wood in the east via a 120km route.
The project, which has been almost 40 years in the making, will be completed in 2019 but London's commuters will be able to start using much of the new Crossrail network from the end of 2018.
Plans for a new railway across London date back as far as the 19th century but Crossrail, in its present form, was first mooted as part of a study into London's railway system in 1974.
Following decades of deliberation by successive governments, work on the scheme finally started in May 2009 but it has lived in the shadows of more glamorous infrastructure projects such as the London 2012 Olympics.
At a cost of 14.8 billion ($29.4 billion), compared with the near 9 billion cost of the Olympics and the 1.5 billion price tag for the London Gateway "super port" in the Thames Estuary, Crossrail dwarfs all other recent infrastructure projects in the UK.
Only High Speed Rail 2, which the Government estimates will cost 43 billion, will be bigger, if it receives the go-ahead to start construction in 2016.
As Elizabeth bores into the London clay that supports the capital north of the Thames at an average rate of just over 90m a week, the miners behind are slotting into place 30cm thick concrete segments, which will form the walls of the tunnel.
"It's a bit like Ikea flat pack furniture," says deputy construction manager Will Jobling as a miner hammers large dowels into the sides of the cement blocks to make sure they slot together.
Jobling is one of more than 8000 people working on Crossrail.
It is estimated that the equivalent of 55,000 fulltime jobs will have been supported throughout the UK during the course of the 10-year project.
Two-thirds of the Crossrail network will use existing rail track, but 40km of new tunnels are being constructed beneath some of London's most built-up areas, including Bond St, Paddington and Tottenham Court Rd, using eight tunnelling machines.
Seven of the German-built machines, which cost about 10 million each, are in operation, and the eighth will start work early next year.
The machines, which are the length of 14 London buses lined up end to end, have to weave a precise path between sewers, power cables and water mains.
Extensive surveys were done on existing infrastructure before work began but the machines are fitted with lasers to ensure they don't hit a power supply line and plunge large parts of the capital into darkness.
There are other obstacles, too. Near Liverpool St, a near 500-year-old graveyard was unearthed, which is believed to hold the remains of as many as 4000 people - many of them from the notorious Bethlem Hospital, the world's first lunatic asylum, better known as "Bedlam".
Workers also had temporarily to drain part of Royal Docks to expand a Victorian tunnel for the project.
Crossrail is being financed through a variety of sources, including a 4.7 billion grant from the Department for Transport. London businesses are contributing through a Crossrail supplement on their business rates, while passengers will help pay off the debt raised by Transport for London once the trains are in operation.
Private companies such as the Canary Wharf Group and Berkeley Homes, which will benefit from the project, have also contributed.
Crossrail chief executive Andrew Wolstenholme says the new railway line will be delivered "in time and on budget" and will generate a 42 billion economic benefit to Britain.
Crossrail has provided employment for construction workers and engineers during a difficult time for the industry and has finally succeeded in winning public and political support.
In May 1994, the first Crossrail bill submitted to Parliament was rejected on the grounds that the business case had not been proven. Now "Crossrail 2", a separate line which could connect areas to the southwest and the northeast of London, is under consideration.
Wolstenholme, who previously oversaw the construction of Heathrow Terminal 5, says "lessons should be learnt" from the torturous process that delayed Crossrail for decades.
The construction industry is riding high after the success of the Olympics, he says, but companies need greater certainty over future projects if they are to keep investing and developing the skilled workforce required to pull off such engineering feats.
A more stable pipeline of work will also help to bring down the overall costs of large-scale infrastructure projects, he says.
"UK plc is right at the top of its game in delivering these major works," he says.
"What we need to do is find ways to bring the pipeline forward ... so the industry is presented with a continuous [flow] of major projects.
"One of the reasons why we are always talking about the costs of doing things in the UK compared to the Continent is because we don't have that continuity of these major projects."