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"The Bathtub" refers to the underground foundation area at the site of the World Trade Center and accompanying buildings in New York City. The term bathtub is a bit of a misnomer, as the area does not hold any water; rather the purpose of its design is to keep water out. The name is more so used to describe its shape of a deep basin with high walls, like a bathtub.
The Bathtub encompasses a large, roughly rectangular excavation down to bedrock surrounded by reinforced concrete walls, intended to serve as dams to prevent water intrusion from the nearby Hudson River (North River). The World Trade Center site was located on man-made water-clogged landfill that had accumulated over centuries, providing an extension of land out onto the Hudson River from the original Manhattan shoreline, with bedrock located 65 feet (20 m) below. Manually removing water from this area would have severely altered the water levels surrounding the World Trade Center site and thus jeopardize the foundations of nearby buildings, causing them to sink. This is why the Bathtub method was used.
The Bathtub contains a 16 acres (65,000 m2) site, including seven basement levels, the downtown terminal of the Port Authority Trans-Hudson rapid transit line, and the preexisting New York City Subway's IRT Broadway – Seventh Avenue Line (1 train). The South Tower of the World Trade Center was actually built around the PATH tubes that passed through the foundation area, thus service was uninterrupted throughout the whole of the construction period. The waterproof walls are 3 feet (0.91 m) thick and 70 feet (21 m) high.
The excavated material that was dug up to build the bathtub was again used as landfill to construct Battery Park City, and the same method was also used to construct the foundation area of the Willis Tower in Chicago.
It was feared that the wall might collapse during the removal of debris from the September 11th attacks, endangering workers and possibly compromising other buildings and flooding a significant portion of the subway system. To prevent this, reinforcements were attached to bedrock to shore up the bathtub walls.