The Long Dig.(Gotthard tunnel )

The people of Allmannsweier, Germany, have never really grown used to the giant mechanical worms outside their village. You might say that the worms crept up on them. When the worms first appeared, almost thirty years ago, they were just little machines, a few yards long, built by a local engineer named Martin Herrenknecht. Now they're among the largest land vehicles on earth. Their hangars loom above the tobacco fields south of Allmannsweier like vast white cocoons. "If we had known how much room they would need, we might not have let them come," the owner of the town's upholstery shop told me. (He happened to be Herrenknecht's older brother.) "But they are here. What can we do?"

Allmannsweier lies along the southern Rhine, in the rich bottomland between the Black Forest and the Vosges Mountains. It was founded more than a thousand years ago, by a tribe of Germanic farmers called the Allemanni, and could still pass for a medieval village. Its half-timbered houses are huddled around an onion-domed church, their haylofts now suspended over BMWs. The only other hint that the local economy has changed--that the town is full of engineers--is a tidiness that verges on the obsessive, even by the standards of southern Germany. Every window seems to have a flower box, every traffic circle its freshly painted lines and sans-serif signs. The streams are flanked by neat rows of beeches and poplars, their branches dotted with crows. The birds were protected after the Second World War, one villager told me: "Now we have to shoot them. There are too many."

Over the years, the locals have developed an uneasy, symbiotic relationship with their primary employer. Herrenknecht AG is now the largest maker of tunnelling machines in the world. Its taxes alone have made Allmannsweier wealthy, and about a quarter of the town's seven hundred adults work at the factory. Many of the top engineers grew up in the area, and new local talent is continually fostered by internships, apprenticeships, and a high-school vocational course paid for by the company.

On sunny spring mornings, kindergarten classes sometimes take field trips to the factory, to see the worms and draw them with colored pencils. The machines are assembled under arched steel cranes and in cavernous halls that smell of carbide and ionized air. Their cylindrical sides are painted grub white--the better to glow in the darkness of a tunnel beneath a mountain--and their cutting heads are bright primary colors. The smallest machines, used to dig tunnels for utility lines or water pipes, are as little as four inches in diameter. The largest, used to dig subway, train, and highway tunnels, are almost a third of a mile long and fifty feet wide. They contain some ninety thousand moving parts and weigh more than nine million pounds. Were all the kindergartners in a class to stand on one another's shoulders, like acrobats in a Chinese circus, they wouldn't reach the top.

"It is a nice toy," Herrenknecht told me one afternoon, standing under a twenty-three-million-dollar unit bound for Caracas. "If you want to buy it for Christmas, I give you five per cent off." He patted the worm's steel flanks and grinned up at its blunt teeth. He looked tiny beside it--like one of the short-tempered dwarfs in "The Hobbit," gazing up at Smaug the dragon. He had a barrel chest and hairy forearms, fierce blue eyes, and an owlish head, tufted with gray and white. He stood with his legs spread wide, his hands clenched at his sides, and communicated mostly in nods and shrugs. When he did speak, it was in an oddly clipped English--he'd spent most of his time in Oberschule "shooting the teacher lady with nails," he said--the explosive sound of which he seemed to enjoy. "You must fight to get the stone out of the tunnel. You must fight!"

Herrenknecht, who is sixty-six, believes that few places can't be improved by a good-sized hole. To see a map of his company's projects is to envision the planet as a porous thing--a cosmic loofah--inhabited by an increasingly hivelike humanity. At any given moment, close to a thousand Herrenknecht worms are burrowing under mountains, rivers, and cities on almost every continent. They have tunnelled along the San Andreas Fault, under the Yangtze River and beside the Bosporus, through catacombs in Rome and petrified pilings in Cairo. "We are changing the world," Herrenknecht told me. "We are putting it in tunnels. That is my vision."

Engineers build cities; the rest of us just live in them. According to population figures from the United Nations, this year marks a turning point in human history: for the first time, more people live in urban areas than in rural ones, and the disproportion grows every year. Whether we walk to work or ride a bicycle, commute along river bluffs or via concrete tunnels, our movements are increasingly dictated from afar--spatial equations solved by technicians we don't know, hunched over the planet like eager boys above a model-railroad set.