The primary challenge facing workers below ground is how to remove debris from the WTC's 16-acre basement without tipping over the Center's slurry wall. The three-foot thick slurry wall surrounds the periphery of the basin of the site and extends down roughly 60 or 70 feet, where it is socketed into rock - mica schist. The slabs of the basement's six floors provided the lateral support for the wall, which keeps the Hudson River and surrounding sediments from entering the foundation and basement. When the twin towers of the World Trade Center crashed down, so too crumbled the six supporting floors, "so the slabs are not there, the floors are not there, but the debris is doing what the floors used to do"