They are hard men in hard hats, members of an underground gang whose weapons are picks and shovels. Meet the men who bore for Britain...
Where would the movies be without the Underground? I can never go on it without remembering the nutter with the axe shouting "mind the gap" in Death Line, or the man with the briefcase falling prey to David Naughton on the escalators at Tottenham Court Road in American Werewolf, or the weary face of Walter Matthau in The Taking of Pelham One Two Three. I've always wanted to go down there without a train. That's another item to strike off my wish list.

There aren't any trains at all on the Jubilee Line extension. There are diggers and borers and men with shovels and JCBs and all manner of metalworking equipment, but no trains. A couple of hundred yards out from North Greenwich station, portal to the Millennium, the pools of water are deep enough to sustain a small eco-system of their own. In this elegantly lined burrow, we splash North-west towards the Isle of Dogs. I assume we must have reached the Thames. Bob Storrie shakes his head and laughs. "No," he says, "You'll know you're under the river when you start to see the fish."

It's one of those things about people who do spend their lives making dangerous things safe: they love to rattle the uninitiated. Bob is the Supervising Engineer of the the Canary Wharf-Canning Town section, and has brought 25 per cent of the project to near-completion. The line will run from Green Park in Mayfair, under the river at Westminster, head East and flip under the river three more times to take in the Canary Wharf complex and North Greenwich, where it will swing north and connect the East End with the rest of the world. The 16km-long tunnel is 92 per cent complete, and everything is on-line for the whole shebang to open in March 1998.

Not before time. If you live outside London, you may well think that this is just a local concern, but the implications of the transport links into Docklands are far wider than that. A devastating chunk - about pounds 2bn - of your tax money has been poured into the development of the area over the past decade or so, and most of it will have been wasted if no one can get there. Canary Wharf itself can hold the entire population of Cambridge, and is currently served by a dinky electric train better fitted to Toytown. No other country would expect Noddy and Big Ears to provide transport to one of its primary financial areas: by 1998 it will be in the hands of the big boys at last.

On top of that, millions upon millions are expected to drop in to the Millennium exhibition when it opens at Greenwich. No one seems entirely clear about what the theme park that will rise in the rubble of the old gasworks is going to contain, except that it will be related to time. Personally, I suspect that part of the theming will involve a lot of people looking at their watches while they stand in queues, but it's bound to be a smash hit. Currently, you can get there via the 108 bus, which goes nowhere near any main railway station. Fittingly for the end of a century, London Underground is running a race against time.

North Greenwich is nearing completion, though to anyone uninitiated in building sites it looks like chaos. It is a statistic junkie's dream: 68,000 cubic metres of concrete; 14,000 tonnes of reinforced steel; a station box big enough to fit the Canary Wharf tower (Britain's highest building) in one-and-a-half times "and still have room for a box of sandwiches at the end"; 755,000 tonnes of "muck" (muck, it seems, is a technical term) moved to make it; the longest platforms, at 135m, in Europe; 650 men (the whole project has over 3,000 people working on it) and two tunnel boring machines called Sharon and Tracey required to construct the section; 480 rooms; 42 high-strength concrete columns supporting the ceiling, from which the ticket concourse is suspended by 34 80mm high-tensile steel rods.

Tunnelling was done at ferocious speed: one section, 1.41km long, was completed in 24 weeks, which works out at about eight-and-a-half metres a day. "But," says Bob, "we managed 254m one week." The tunnels themselves are peaceful now, awaiting the arrival of the rails from Canning Town. One can walk the best part of the kilometre or so to Canary Wharf without encountering more than a handful of chaps in hats, eating sandwiches or hosing down the walls in preparation for painting. We stopped and Bob pointed at the ceiling. "We're 40m underground here," he said. I don't suppose anyone on Preston's Road was giving a second thought to what was under their feet.

If North Greenwich has the shiny, sci-fi cleanliness of, say, the tunnels the holidaymakers trundle down on the way to being slaughtered by Yul Brynner in Westworld, then the scene at London Bridge is a good deal more like Quatermass and the Pit. It is down here that you see where HG Wells got his inspiration for The Time Machine: the novel was published in 1895, five years after the Northern Line was completed. This is the world of the Morlocks, a place where the fruit-fed Eloi are herded to be eaten.

There are 500-odd men working down here. One says men because, although women are comfortably represented on the engineering side of things, the manual labour is exclusively masculine. A lot of them, for that matter, have moustaches. One could almost imagine that these hairy, sweating, hard-hatted chaps could be auditioning for the Village People. Except for the lack of lateral obliques and the fact that no member of the Village People could cope with getting that grimy.

Most of the men working down here have come from the Channel Tunnel. Finding enough skilled labour has been quite a problem, as major digging works like this have been few and far between in recent years. "We're having to train quite a few people on the job," says Walter Bermingham, Senior Inspector of Works. "The last big training time was when they built the Victoria Line [finished to coincide with the Queen's Silver Jubilee], and most of the skilled guys are my age. We need some new blood."

Wages for working these hell-fire caves are in the region of pounds 50,000 a year, and the workforce comes from far and wide, mostly the old colliery areas and also from Ireland. What there is not a preponderance of is Londoners. Work goes on 24 hours a day, seven days a week; men work six 12-hour shifts, have three days off during which they either hit the pubs or shoot home to their families, then swap on to the other shift for another six days.

London Underground and its contractors are keeping a tighter rein on their employees than was kept on the Channel Tunnel: while these guys can, and do, indulge in serious drinking bouts, the kind of debauchery that went on around Ashford has never quite hit the hostelries of south London. There was the famous time, for instance, when a chunnel worker had an ear bitten off in a fight, had it sewn to the inside of his thigh to await grafting back to his head, and never reported back to the hospital. Maybe he just undresses in the dark these days, though you'd have thought he'd have a bit of a chafing problem. The hangover-related deaths that plagued the chunnel are also not in evidence here. "There's been a broken leg and a few lost fingertips," says Walter, a man of careful words and 30-odd years' experience, "but no serious accidents, no."

Which isn't to say that there's no drama involved in reaching the faces. Cigarettes are strictly forbidden in this smoggy air, goggles, hard hats and face-masks are issued. We all strap on "self-rescuers" - oxygen masks in yellow carry cases - and are told how to work them. I expect someone to point out the emergency exits over the wings. Everyone has two numbered metal dog tags: one they hang on a board at the top of the adit, the other one they carry on their person. "That way," says Marcus Karakashian, Construction Manager, deadpanning it in true engineering style, "at least they can identify the bodies." I do wish they wouldn't do that.

Thirty metres down in a lift, and the heat is intense. Huge fans blow air down to the faces, but sweat pours off everyone as they paddle through the mud. The tunnels here have been initially completed, and covered with a temporary lining: it looks like the kind of stippled grey harling that decorates the buildings on bad northern housing estates. Huge metal conveyor-belts carry massive loads of mixed muck and stone to the adit entrance, where the muck-lift hauls it to the surface. Someone has painted the Scottish flag on the side of the lift. There's a separate one for people, you see.

A fine mist hangs in the air; figures loom suddenly from the distance, like cars on a foggy night. It drifts around the side of your goggles, coats the inside of your nose, petrifies the hair. We visit the new southbound Northern Line tunnel, which is being built to improve safety in the wake of the King's Cross fire: from 26 June, that branch will be closed for four months and the 50,000 people it carries daily will have to find another way to work. Everything is prepared: Marcus shows us where the old tunnel has been excavated: a hundred- year-old cast-iron tube surrounded on all sides by air. I put my hand on it and it vibrates as a train passes by. I don't suppose that any of the passengers have the first idea that they are only feet away from a JCB.

We walk on until we're under the river. The old and new tunnels will join up here, near the foot of the old bridge, the one which fell down and whose foundations have caused significant engineering challenges. Here, tunnelling is going on by hand and pony boys wheel carts of muck back toward the surface; it's a scene from the Industrial Revolution. Here, also, it smells different: sort of stagnant and river-like. I ask Walter, thinking I'm being intelligent, if this is because the nature of the soil has changed. "No," he says. "This is the bit the men use for a lavatory."