The new subway tunnel connecting Downtown to the North Shore is finally getting under way.

The $10 million German-built tunnel boring machine actually got to work on Tuesday, but yesterday was when Port Authority engineers and contractors showed off their latest project.

When it's done in three years, there will be two 2,400-foot-long tunnels snaking beneath the Allegheny River, extending light-rail service to the fast-developing areas around PNC Park and Heinz Field.

Yesterday, the 500-ton machine made by Herrenknecht AG was taking a breather as workers installed giant rings behind it, which its 20 hydraulic arms will push against to keep moving through the soil.

The white, cylindrical drill, sporting the signatures of those who helped install it, sat in a concrete-lined pit 55 feet beneath street level near PNC Park.

Beginning Monday, it will start churning again and spend the next three to four weeks moving about 100 yards forward and slightly downward as it heads toward the bank of the Allegheny, said Winston Simmonds, Port Authority engineering manager.

Once it reaches that point, drilling will halt for another three weeks or so while workers finish installing the trailing gear that extends from the rear of the machine -- the control cab, the hydraulic and electrical lines that power the machine, and pipes that pump a water-and-clay mixture to the front of the cutting heads to help them slice through the earth, and then transport earth and stone back to the surface.

Because the bentonite clay that makes up the slurry is a valuable commodity, contractors have built an entire structure on the North Shore to separate the clay from the excavated earth so the clay can be reused.

Sometime in March, the machine will start "continuous mining," as Mr. Simmonds put it, moving about one foot an hour.

Asked yesterday how an observer could tell the drilling machine was at work, tunneling expert Steve Minassian pointed to a ring near the front and said, "That'll move about half an inch to an inch per minute. It's not going to spin like the engine on a 747 or the Space Shuttle."

At no point will the machine encounter water. It will burrow at least 20 feet beneath the Allegheny riverbed, and even deeper toward the center of the river.

After the machine emerges into a receiving pit underneath Stanwix Street, Downtown, it will be turned around to dig a second parallel tunnel back to the North Side.

The $435 million project is designed to capitalize on the fast-developing area around PNC Park and Heinz Field, ferrying people to that area for games and other entertainment, and bringing commuters into the city to work.

It includes plans for a new Downtown station and two new North Side stations -- an underground station near PNC Park and an above-ground station near Heinz Field.