(FORTUNE Magazine) – The bustling construction activity in the photo at right shows just a portion of what Bostonians, with a mixture of pride and annoyance at all the detours, call the Big Dig. Known properly as the Central Artery/Tunnel Project and expected to cost $10.8 billion by the time it is finished in 2004, it is the largest and most complex piece of highway construction in the world today. Contractors are tearing out, replacing, widening, and extending two major interstate highways in, near, and under one of America's liveliest city cores.

The highways I-90 and I-93 will intersect at a giant new interchange being built (lower left in the photo). To make a link from there to the new Ted Williams Tunnel under Boston harbor, powerful machines are literally shoving tunnel sections under heavily used railroad tracks near South Station. They do so without disturbing electrification work on the railroad, an unrelated project that will soon bring high-speed Amtrak service from New York. In the most ambitious part of the Big Dig, machines are excavating tunnels to replace the Central Artery, a traffic-clogged elevated expressway that for 40 years has blocked Boston's view of its waterfront.

Along with truckloads of designers' blueprints, the plan took years of political engineering. To authorize 70% federal funding, Congress had to override a 1987 veto by President Reagan. It helped that the late Tip O'Neill of Massachusetts was speaker of the House when congressional support was being lined up. "We were very lucky to have him and Teddy Kennedy," recalls Frederick Salvucci, the state's secretary of transportation at the time. Salvucci, who worked on the elevated road as a teenager, calls it "a scar on the face of Boston." He led the drive to include its dismantling in what became the Big Dig.

Some had argued that replacing the Central Artery would snarl downtown traffic for years. But special machines, operated within a stone's throw of office workers on a lunchtime stroll, can drill trenches for tunnel walls close to buildings without disturbing their foundations. The walls temporarily support the elevated Central Artery as earth removal goes on below it. That's just one of the Big Dig's advanced construction techniques, which can be seen on the pages that follow.