and the Bergen Hill tunnels, west from the Weehawken Shaft to the tunnel portals on the west side of the Palisades. The tunnels were built with drilling and blasting techniques and tunnelling shields, which were placed at three locations and driven towards each other. The shields proceeded west from Manhattan, east and west from Weehawken, and east from the Bergen portals. Under the river itself, the tunnels started in rock, using drill and blast, but the strata under the river was pure mud for a considerable depth. As a result, this part was driven under compressed air, using 194-ton shields that met about from the Weehawken and Manhattan portals. The mud was such that the shield was shoved forward without taking any ground; however, it was found that the shield was easier to steer if some mud was taken in through holes at the front, since the mud had the consistency of toothpaste. After the tubes had been excavated, they were lined with segmental cast-iron rings, each weighing 22 tons. The segments were bolted together and lined with of concrete.The two ends of the northern tube under the river met in September 1906; at that time it was the longest underwater tunnel in the world. Meanwhile, the John Shields Construction Company had begun in 1905 to bore through Bergen Hill, the lower Hudson Palisades; William Bradley took over in 1906 and the tunnels to the Hackensack Meadows were completed in April 1908. Opening and use The tunnels opened November 27, 1910, when the New York Tunnel Extension to New York Penn Station opened. Until then, PRR trains used the PRR main line to Exchange Place in Jersey City, New Jersey. The New York Tunnel Extension branched off from the original line two miles northeast of Newark, then ran northeast across the Jersey Meadows to the North River Tunnels and New York Penn. The tunnel project included the Portal Bridge over the Hackensack River and the Manhattan Transfer interchange with the Hudson and Manhattan Railroad (now PATH). The opening of the North River Tunnels and Penn Station made the PRR the only railroad with direct access to New York City from the south. In 1967 the Aldene Plan was implemented, allowing trains of the floundering Central Railroad of New Jersey (CNJ) and Reading (RDG) to run to Newark Penn Station, connecting to PRR and PATH trains to New York. The PRR