Tracy the tunnel engine

Sean Thomas goes mining on the Jubilee Line

The place looks like any other construction site: a wasteland of rusty skips, razor wire, and shallow pits of oily water. It is only when you approach the edge of the central hole - a massive void that will one day be North Greenwich Underground s tation- that you realise how breathtaking the endeavour is. Experts estimate this single excavation could hold more than 3,000 London buses. Teetering on the muddy precipice, I believe it.

I am here to work a shift with the men and women building London's Jubilee Line extension. It is planned to run from Green Park, through Westminster, to Bermondsey and Docklands, thereby linking Canary Wharf with the Western world. Costing about £200m per mile, the JLE is one of the most expensive transport projects undertaken anywhere on the globe. If such sums seem ludicrous to us surface-dwellers, it can seem penny-pinching to the miners hewing through the earth, 40 metres below.

"I've worked on a lot of the big projects," says a tunnel engineer David Shepherd, as he guides me down a spiral steel staircase to the entrance of the northbound borehole. "The Channel tunnel was by far the best job. It was state-of-the-art. Nothing wastoo much money. Here it's more hard-headed."

We begin the subterranean journey to the clayface where the tunnelling is taking place and, as we descend beneath the Thames, the air gets staler. I can hear the rumbling throb of the Big Machine. Above me is a fat, black pipe, running along theroof of the tunnel. This conducts fresh air to the men at the end. Clutching my legally required respirator, David Shepherd guides me to the action.

It's quite a distance. On the way I stop and watch a blue-overalled German miner, an employee of the Continental half of the McAlpine/Freytage-Bachy consortium, as he squirts grout behind one of the concrete castings. Farther on, another man is valiantlystaunching a water leak - at this point the river is just 20 feet above our us. Once past the whirring water pump we see journey's end.

Huge train-loads of slurry and muck are being expressed past us, back to the surface. The soil seems to glisten. Much of the land in the north Greenwich peninsula has been contam-inated by the chemical plants and gasworks previously sited there - even this far underground the rich clay is shot through with cyanide, arsenic, mercury and sulphur.

As we round a corner I hear shouts. The concrete cavity shudders. I smell oil and sweat, and suddenly there she is: Tracy. The miners of the JLE have nicknamed their huge Canadian-designed tunnelling machines after certain types of girl. This one is called Tracy because she's heading for Essex, she's usually boring, and with a bit of luck she'll go all the way. Tracy is a big girl: 100ft-long hardened steel cylinder, packed with electronics, guided by laser beams, equipped with ground probes and gyroscopes, and tooled with a dozen big tungsten carbide drillheads.

Up in the cab, I stand with Tracy's driver, Jim Gilbain, and start my day's work. As Jim explains, the faster we go or the trickier the going, the hotter it gets. Seeing as it feels about 3 million degrees up here in Tracy's tiny brain, we must be drilling through some pretty rough stuff, right? Jim looks at me pityingly: "Wrong."

The day progresses. Tracy edges forward, chewing her way through clay and shale. Every half hour she halts, and the men start fitting the concrete segments behind - constructing the tunnel lining. There are 10 men on this team. Each team works an eight-hour shift, five shifts a week. Tunnelling con-tinues through the night Monday to Friday; weekends are reserved for maintenance. David Shepherd explains how every man has a different task. Some are bolting into place the curved concrete panels. Others aredealing with the spoil, laying new tracks or fixing the lights. And others are keeping Tracy happy with her favourite tipple: 10,000 volts of electricity.

Chatting to the miners, it is clear that the profession of tunneller is a clannish one. A lot have worked together on past projects, and many hail from the same parts of Ireland, Scotland or Cornwall. One reason these jobs are so tribally cherished is that they can bring in £40,000 a year. But it's a dangerous job: eight workers died on the British end of the Chunnel.

Six hours later, I re-emerge into the winter sunshine. Hailing a cab, I watch as the cranes swing another load of concrete segments on to a lorry. Critics say the money should be spent on existing Tubes, buses, or bicycle lanes. They are wrong. The JLE is not only an engineer's dream, it is much needed: my cab back to Bond Street took an hour.