The Port Authority of New York and New Jersey acquired permission to build the Lincoln Tunnel from New York and New Jersey soon after the completion of the Holland Tunnel. At the time the project was called the Midtown Hudson Tunnel and proposed to connect Weehawken, New Jersey to midtown, Manhattan, with a structure 1.5 miles in length, built under the river bed of the Hudson. The idea for Lincoln Tunnel was first proposed by the Port Authority in 1930, and later approved in 1933 when Robert Moses, chairman of the New York State Emergency Public Works Commission, managed to secure the necessary funds for the Port Authority. Othmar Amman the chief engineer at the Port Authority consulted with Ole Singstad on the Lincoln Tunnel Project to learn from the experience Ole Singstad had gained during his employment as chief engineer of the Holland Tunnel. The construction on the Lincoln Tunnel was done by men who were often referred to as sandhogs because of the conditions that they worked in. To build the Lincoln Tunnel the sandhogs had to assemble hundreds of massive iron rings, on site. These rings weighed approximately 21 tons, and formed the lining of the tunnel; this type of work was extremely dangerous and challenging. The air pressure was maintained at a very high level in the tunnel during construction to hold back seepage. The men had to endure the arduous and uncomfortable process when entering and exiting the tunnels. For the workers to be able to adjust to the high pressure environment under water, they had to enter the airlocks in increments, with each adjoining lock having higher pressure than the lock that came before it. This process would be repeated on the way out of tunnels, by decreasing the pressure in small decrements. The sandhogs entered the airlocks one at a time; having reached their destination, they had to work fast to avoid the getting a type of compression sickness called the bends. One of the most challenging aspects of engineering the tunnel was the fact that work was in progress from both ends of the Hudson River: this meant that the tunnel being built by men on the New Jersey side, had to align perfectly with the tunnel being built by the workers on the New York side. The Lincoln Tunnel consists of three separate tubes, each of which were completed at different stages in New York City's history. The first tunnel was opened to traffic on December 22, 1937 and only allowed one lane of traffic in each direction. On February 1, 1945, the north tube was opened, and allowed two-lane traffic in both directions. The third and southern tube began construction in 1951 and was complete on May 25 in 1957.