once constructed, the bathtub walls relied on the presence of the basement floors of the wtc to give lateral support. when these were partially destroyed following the collapse of the twin towers during the september 11 attacks, it was feared that removing the resulting debris pile could weaken the walls and cause them to fail, endangering workers and possibly compromising other buildings and flooding a significant portion of the subway system. during the recovery and clean-up operation, dangerous degradation of the walls was discovered, prompting an emergency operation to install tiebacks to the bedrock to shore up the bathtub walls. excavation of a new bathtub was conducted between 2006 and 2008, with the new bathtub reaching underground.