The term, formerly familiar mostly to architects and others with knowledge of, or involved in the construction or maintenance of, the site, became more widely known after the collapse of the Twin Towers and other buildings as part of the September 11, 2001 attacks.
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 Bathtub contains a 16 acre (65,000 m²) site, including seven basement levels, the downtown terminal of the Port Authority Trans-Hudson rapid transit line, and a preexisting New York City Subway line. The waterproof wall is three feet (~1 meter) thick and 70 feet (~21 meters) high.
It was feared that the wall might collapse during the removal of debris, 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.