The river's edge is marked by a crude blue line and the word "Allegheny" spray-painted on a makeshift wooden walkway.

But the mighty river is nowhere in sight. It is about 50 feet directly overhead, hidden by a concrete ceiling 22 feet high and 3 feet thick that has at least 25 feet of earth atop it.

For all the millions who have plied the Allegheny River on watercraft or crossed its many bridges, only a few hundred can say they've ventured beneath it.

"This is history," remarked Dave Whipkey, Port Authority spokesman, who has been part of a dozen or so tour groups that have visited the twin tunnels bored beneath the river for a subway extension to the North Shore, scheduled to open in 2011.

Whether the rather exclusive underwater fraternity is to be expanded remains in doubt, as the Port Authority searches for $117.8 million to cover cost overruns on the half-built North Shore Connector.

Yesterday, though, the project continued apace, with compressors, jackhammers, cranes and trucks providing the sound track for 238 workers who tromped through icy mud at the North Shore and Downtown construction sites.

The new subway station near PNC Park looks to be a long way from ready for customers. Reaching the underground level requires tricky descents on two narrow ladders.

The sprawling platform where passengers will board trains still shows its lattice of reinforcing rods on one end -- some of the concrete has been poured at the other end and is covered in plastic sheeting.

The towering walls on either side of the platform are draped with a black waterproofing membrane "similar to a swimming pool liner," said the connector's project director, Keith Wargo, who led a small tour yesterday.

Overhead power lines, tracks and plinths -- the concrete risers on which the tracks are laid -- have yet to appear.

But the subway extension also is too far along, too vast and imposing, to look like a project that might be in terminal peril.

From the top of the adjacent West General Robinson Street parking garage, one can see the planned path once the line rises from underground just after the PNC Park station and moves toward an elevated station at Heinz Field.

And there are the tunnels, the engineering marvel of the project, gouged out with the brute force of a 500-ton boring machine and the high-tech finesse of a laser-controlled guidance system. Work on the tunnel contract is about 80 percent complete, Mr. Wargo said.

The tunnels are sheathed in concrete -- prefabricated rings 4 feet long and 22 feet in diameter made up of seven sections each that were bolted together as the tunnels were dug.

The giant boring machine has completed its job, been dismantled and is in the process of being shipped off the site.

From their starting point near PNC Park, the tunnels take a gently curving route toward Stanwix Street, Downtown. One passes beneath the Equitable Resources building on the North Shore.

At their lowest point, the twin tubes are about 100 feet below the normal surface of the river, which is 710 feet above sea level. The top of the 3-foot-thick concrete that makes up the tunnel rings is at least 25 feet below the river bottom.

The next contracts to be awarded are for "train systems" -- tracks, electric power supply, signals and communications equipment -- and escalators. The combined cost of the low bids is $88.3 million, and yesterday, even as the "Bore to the Shore" continued, Port Authority was not certain where -- or if -- it would get the money.