AMSTERDAM -- Seventeenth-century masons built Amsterdam on a foundation of wooden poles planted in soggy, sandy ground, leaving behind a beautiful architectural museum -- but one with walls prone to sinking or crumbling without warning. So how do you dig a subway under it?
Construction of a new "North-South" line for this city of canals and rivers began in 2003, and is presenting Dutch engineers, famed for their ingenuity in keeping this waterlogged nation dry, with devilish challenges.
"The politicians told us: 'We want a subway, we're prepared to pay for it and accept some disruption, but the one thing we absolutely don't want is any damage to the city,' " said Johan Bosch, the project manager.
"We need a system so that if things don't go as expected, we don't find out after the damage is irreparable."
The solution: 7,000 mirrors hung in clusters of three on buildings along the 2.4 miles of the route that's underground. Measuring devices shine infrared beams onto each mirror once an hour, measure the reflection, and feed data into a central computer.
After triangulating, the computer raises the alarm if any building shifts more than 0.5 millimeters in any direction. A millimeter is the thickness of a paper clip.
The system, unique on such a large scale, has told townspeople something they may have guessed but couldn't know for sure: Theirs is a city in motion.
"We now know that whole segments of the city move by themselves, a number of millimeters over the course of a season," Bosch said.
Scheduled for completion in 2013, the $2.4 billion project extends 5.9 miles in all and will transport an estimated 200,000 people daily, adding a new dimension to Amsterdam's traffic of bicycles, trams, cars, taxis, buses, and boats.
The biggest technical challenge is building a subway stop directly beneath the city's main train station -- a landmark 19th-century building -- while it remains in use.
A quarter-million travelers pass through Centraal Station every day and few of them realize construction is happening beneath their feet.
"It's probably better that people don't see this," said Bert van de Zande of Strukton, the contractor responsible for this part of the project, pointing to I-beams supporting the building's main columns.
Strukton workers will dig a 60-foot-deep trench under the station, and connect it to the Ij River, which flows directly behind.
"It'll be the world's most expensive covered canal," Van de Zande said. When all is ready, a segment of concrete tunnel 450 feet long will be buoyed with air like a submarine, floated into the trench, and then allowed to settle gently into place.
This fall, two drills, each 23 feet in diameter, will begin tunneling through the city like huge, metallic moles pulling the contraption that lays concrete tunnel walls.
Archeologists can't wait for the tunneling to begin at the Damrak, which was once the harbor where the Amstel River met the Ij, leading out to the North Sea. In the 1600s, countless ships returning from the East Indies docked there, making Amsterdam -- the dam on the Amstel -- one of the world's wealthiest cities.
The initial segment where the drills will descend into the earth is being built using the "caisson" method. The sealed segment, or caisson, is constructed above the spot where it must go. Workers operate in a pressurized air pocket directly beneath it, using hoses and pumping mud upward through a pipe so that the caisson slowly sinks.
Because the streets of Amsterdam are below sea level at high tide, the air pocket is increasingly pressurized as the caisson descends to a final depth of 82 feet.