Viewed from its starting point, the new Port Authority tunnel looks like an ivory umbilical cord swooping beneath the Allegheny River.

How much new life it will nourish between Downtown and the North Shore remains to be seen, but there is no doubt how impressive its engineering progress has been.

This first of two 2,240-foot transit tunnels has now passed the halfway mark in its march toward Downtown, as a $10-million German boring machine chews its way through 35 to 45 feet of soft rock each day, 25 feet below the Allegheny's riverbed.

As of this weekend, the tunnel had progressed more than 1,300 feet from its launch pit near PNC Park, and Port Authority officials expect it to break through the earth into the receiving pit beneath Stanwix Street by the middle of next month.

Like an inchworm, the 500-ton Herrenknecht boring machine moves forward by pushing off from the concrete subway tunnel segments that are steadily laid down behind it.

After its 24 giant hydraulic cylinders push the drill forward about five feet, the cylinders retract into the machine, and workers install the next tunnel ring behind it. Then the process starts all over again.

"They stop mining just long enough to erect the tunnel segments" before moving on again, said Keith Wargo, the Port Authority's North Shore project director, during a recent tour of the tunnel.

The erection of the tunnel walls is an intricate marriage of high-tech machinery and human muscle and brainpower.

There are seven precast concrete segments in each 4-foot-long ring of the tunnel. The pieces, made by A.C. Miller Concrete Products in Blairsville, 40 miles east of Pittsburgh, are fit together like an oversized interlocking puzzle.

The segments, stacked like Pringles chips, are hauled to the drilling area on rubber-tired vehicles, where an erector arm grabs onto a screw threaded into the middle of each segment and lifts it into position.

The arm is controlled by a worker manipulating a fat toggle switch on a control board. He shifts each segment up, down and sideways with the help of other employees, who lean forward with folding rulers to make the final measurements and shout directions at him through the din of the drilling operation.

The workers then use pneumatic drills to bolt each ring to the next one. Snug between the rings are rubberized seals to keep out moisture.

Although the segments are bolted to each other, they aren't anchored to the earth around them. Instead, the boring machine creates a tube that is slightly larger than the subway tunnel, and the 3 or 4 inches of space around the outside of the tunnel walls is filled with concrete grout to give the tunnel stability.

The tunnel rings are tapered, with one side slightly shorter than the other. That way, if the tunnel needs to curve, the workers can put a series of the shorter sides together. When the tunnel is moving straight ahead, the workers simply alternate the short sides from one ring to the next.

By the time the project is done, there will be 1,140 rings in place in the two tunnels. They will be filled in at the bottom with a smooth concrete pad for the rail car tracks.

The glistening subway tube that now snakes behind the drill has remained relatively clean during construction because of the particular system the Port Authority's contractors are using on this job.

Some drilling jobs remove the excavated dirt and stone on a conveyer belt through the middle of the tunnel. But this project uses big pipes to pump a clay-and-water mixture known as slurry to the front of the drill, and then take the slurry and soil mixture back to the surface on the North Shore.

The slurry, which uses a clay known as bentonite, serves two purposes. As it is pumped in front of the drill head at a pressure of 30 to 35 pounds per square inch, the slurry helps keep the earth from collapsing toward the machine. The milkshake-thick mixture also lubricates the soil as the 48 drill teeth rotate at one to two feet per second.

Once the excavated material has been pumped back to the surface, it enters a blue and white separation plant that sits opposite PNC Park. Inside the building, the soil is screened out and hauled away, and the slurry is reprocessed to be sent back to the front of the machine.

The tunneling already has generated 20,000 cubic yards of earth and stone, or about 1,000 truckloads, all of which has been hauled to the former LTV steel mill site in Hazelwood.

Since the tunnel started 55 feet beneath the surface and has moved even deeper as it burrowed under the river, how does it know where it's going?

The Port Authority's contractor, North Shore Constructors, started by mapping the tunnels' route using surface surveying equipment. That information was then transferred to a computerized laser device in the launch pit that fires a beam down the tunnel until it reaches the boring machine.

Using the laser's directional and length measurements, a display shows the control cab operator exactly where the boring machine is and where it's headed, and compares that with the route it should be following, allowing minor adjustments as it moves along.

As the boring machine plows toward its target, work is proceeding in the opposite direction on the North Shore.

The subway lines will continue running underground from the launch pit area to a new station beneath the Sports and Exhibition Authority parking garage on West General Robinson Street, and then on to the point where they will pierce the surface east of Heinz Field.

The boring machine will not be used to create that stretch of the tunnel, though. Instead, workers have dug down from the surface to make a trench for the subway tubes.

The station beneath the parking garage already has its walls and most of its roof, and the floor is being installed now, Mr. Wargo said. The station shell and the tunnel sections on either side of it should be completed by December.

Work on the rest of the route, from where the tracks will emerge from the ground and then keep rising to run on an elevated platform to the final station near Allegheny Avenue and Reedsdale Street, has not yet started.

On the Downtown side, the Port Authority will build a new station under Stanwix Street, and has not begun that work yet, either.

When the 30-foot-long boring machine breaks into the receiving pit Downtown next month, it will slide onto steel plates and be turned around with hydraulic jacks so it can start drilling back under the river in the opposite direction. It is scheduled to arrive back at its launch pit in December. The entire connector is scheduled to begin passenger service in 2011.