Planners for the complex wanted the Trade Center to be a hub for culture and entertainment - not just a place of work. To facilitate this end, a number of amenities were included, the largest of which was an underground concourse. @ over 300,000 ft2 it was the largest mall in Manhattan with over 80 stores and managed by the Westfield Group.

It would later play a vital part during the attacks by allowing thousands to safely escape the towers under the cover of shelter from falling hazards above.

Source: https://www.youtube.com/watch?v=n-0dCt-EQOg