Any time we don''t have crowding during rush hour, there''ll be a receiver sitting in the mayor''s chair and New York will be a ghost town. Why, they talk about the rush hour and the crash and noise. Why, listen, don''t you see that''s the proof of our life and vitality? Why, why, that is New York City! --Mayor Fiorello LaGuardia, 1943 1. The Subway that Time Forgot New York, New York Something impossibly big and powerful was moving beneath the city. In a portable office at a construction site in Chelsea, I could already sense its presence as a low rumbling rising through the soles of my rubber boots. As I rode a steel cage fifteen stories down a circular abyss, the physical shudder became an all-encompassing roar. The elevator touched down on the floor of a high-ceilinged chamber blasted out of gray rock, where a makeshift factory was abuzz with activity. Beside me, an arc welder''s torch cracked and sizzled; a crane hoisting stacks of precast concrete overhead emitted piercing beeps; a construction train juddered to a halt and shook a couple of dozen tons of broken rock onto a conveyor belt.

Underneath it all, the earth-rattling roar never abated. At the chamber''s north end, where the ceiling lowered into parallel tubes as smooth and round as twin shotgun barrels, the tunnel on the right thrummed like Hades'' didgeridoo. It was hard to shake the feeling that a living thing had been unleashed, and was gnawing its way through the bedrock of Manhattan. "It''s up ahead, about a thousand feet!" my guide yelled into my ear. Rich Redmond, a consulting engineer for New York''s Metropolitan Transportation Authority (MTA), and the man responsible for day-to-day operations on this site, walked ahead of me into the tunnel. I squinted into the distance, but the tube darkened and narrowed at the limits of my vision. As my mind conjured visions of sandworms and minotaurs, I followed Redmond into the gloom. We splashed through murky water that purled around the ties of a single railway track, a rivulet constantly fed by groundwater seeping through the concrete walls.

Ahead of us, the end of the tunnel was plugged by a vibrating disc fitted with red and green lights. Just as moving forward was becoming physically painful--it sounded like the world''s biggest wood chipper was grinding its way through a petrified forest--the roar stopped. Redmond paused and shouted back: "They must be done with the shove. If you want to see it, now would be a good time!" Quickening our pace, we came to the end of the line: a floor-to-ceiling framework of horizontal walkways and vertical ladders, draped with a cat''s cradle of coiled wires and dangling tubes. The monster at the end of the tube was a Herrenknect Double Shield tunnel-boring machine--a TBM for short. "This is the trailing gear," explained Redmond, as we clambered aboard the back end of the machine. "It''s about three hundred and fifty feet long; it houses the ventilation, the pumps, the electrical systems." Custom-made for the job at a workshop in Germany, the thousand-ton TBM was so massive that it arrived in New York in three separate transatlantic shipments; reassembling it, once it had been lowered down the excavation shaft, took two and a half months.

The machine''s immediate goal was the cavern of a new station at Thirty-fourth Street. From there, it would veer from Eleventh Avenue, boring under Penn Station''s Amtrak tunnels, before connecting to the 7-line platform at Times Square. The 7-line extension, as the project was known, would ultimately allow New Yorkers to commute to jobs at the Javits Center and elsewhere on the far West Side. Weaving through pipes and beams, we came to the TBM''s nerve center, a small booth where an operator in green coveralls sat in front of computer screens monitoring the monster''s progress. "The screens show the thrust, applied pressure on the rock, and the torque of the machine," said Redmond. Guided by GPS, lasers, and radar, the operator was responsible for adjusting hydraulic levers to keep the machine''s cutting-head on course. Though it is made of metal and powered by electric motors running on 13,000 volts, the tunnel-boring machine moves like a living organism. Its rotating cutting-head resembles the circular maw of a lamprey eel, with forty-four spinning discs of alloy in the place of pointed teeth.

Bracing itself against the tunnel walls with two convex grippers, the machine uses pistons to force the cutting-head forward; the crushed rocks fall into chutes behind the discs and are ferried away by a central conveyor belt. When the TBM has completed a five-foot "shove" into solid rock, the grippers are released, and, like a colossal caterpillar, the machine shifts its entire thousand-ton bulk forward by lifting and dropping its multiple feet. Climbing a ladder, Redmond and I ducked beneath the ceiling of the tunnel, and scuttled toward the business end of the TBM. Three workers in hardhats and overalls were on their sides, grunting and cursing as they tightened the bolts on a curved, five-foot-wide segment of precast concrete; a half-dozen such segments made one complete circle of tunnel wall. On a good day, said Redmond, the machine could complete a shove in half an hour, lengthening the tunnel by 60 feet over three eight-hour shifts. On a bad day, when unexpected conditions were encountered, everything ground to a halt. As we backtracked through the trailing gear, Redmond reached into an open rail car and passed me an arrowhead-shaped piece of broken rock, flecked with glints of mica. "This is the muck train," he yelled.

"Muck is vernacular for mined rock. You''re holding a piece of pure Manhattan schist in your hands." The TBM makes short work of the large-grained whitish-gray rock, as it does granite, which accounts for much of the bedrock undergirding the skyscrapers of New York. In less predictable ground, however, other techniques are necessary. "We ran into water-bearing glacial material at the very start of tunneling," said Redmond. "We had to drill holes and run tubes from the surface, fill them with liquid brine, and use a freezer plant to bring them down to thirty below zero. They basically turned the soil into a giant block of ice--which made it solid enough to mine." Once the freezer was turned off, grout was injected between the concrete walls and the rock, sealing the tunnel against seeping groundwater.

All around us were sandhogs, New York''s legendary urban miners, broad-shouldered, stocky men in jeans and safety vests thoroughly begrimed with gray slurry. The sandhogs refer to the tunnel-boring machine as "the mole"--a nickname that is not entirely affectionate. For generations, the power behind the cutting-head was almost entirely human. Since laying the foundations of the Brooklyn Bridge in 1872, the sandhogs have dug every important sewer, water, and train tunnel in the city. These storied laborers, many of them Irish, African American, and Italian, dug the Lincoln and Holland tunnels, shored up the collapsing Trinity Church, and excavated the city''s subway lines, blasting with dynamite, tightening bolts with 75-pound wrenches, often clearing away muck with their bare hands. Working in pressurized tunnels when they dug beneath New York''s riverbeds, they died by the dozens in cave-ins and catastrophic blowouts. The sandhogs are still a force to be reckoned with beneath the streets of New York. An Irish flag hangs at the entrance to the excavation shaft, and before digging on the 7-line extension could begin, the city''s new Roman Catholic archbishop paid a visit, tracing the sign of the cross over bowed hardhats.

Automation, which began when New York''s first TBM was lowered into the shaft of the Third Water Tunnel in 1970, has been both a blessing and a curse. With the coming of the "mole," the most dangerous part of the sandhogs'' jobs, digging into potentially unstable ground with picks, shovels, and percussion drills, was eliminated--but so were a good number of jobs. At the turn of the last century, it took almost 8,000 men, working for two dollars a day, to excavate New York''s first subway lines. Today''s sandhogs are paid far more--over $100,000 a year, with benefits--but only a few dozen are needed on each eight-hour shift. The mole, the steam-powered hammer to the underground miners'' John Henry, has turned out to be the biggest, toughest sandhog of them all. Around the world, hundreds of such machines are now at work, chewing through geology in an unprecedented push to increase human mobility. In Beijing, Madrid, Delhi, and Los Angeles, TBMs are drilling beneath the feet of urbanites, in one of the most astonishing bursts of transit infrastructure building in decades. As the planet enters a new phase of urbanization, cities are looking to advanced transit systems as the way out of congestion, pollution, and economic stagnation.

The mole may have taken the danger, and thus the glamour, out of tunnel digging, but it is allowing cities to build new lines on time and on budget, with no loss of human life. The fact that New York--which for decades stood alone among great cities in that its total track mileage was actually decreasing--is finally digging a new tunnel, is a sign of the times. After half a century of freeway building, the subway is back. If all goes well, Redmond explained to me as we rode back to the surface, the work would be completed some time late in 2013. According to its critics, the 7-line extension, which will add exactly one stop to the existing network, is a "subway to nowhere"--a waste of precious resources in recessionary times. It was intended to serve a stadium for the 2012 Summer Olympics, but when New York lost the bid to London, plans for a second stop for Hell''s Kitchen--a neighborhood sorely in need of another stop--were dropped. The line wil.