During the September 11, 2001 attacks, two aircraft hijacked by 10 al-Qaeda terrorists flew into the North and South Towers of the World Trade Center in New York City, causing massive damage and starting fires that caused the weakened 110-storey skyscrapers to collapse. The destroyed World Trade Center site soon became known as "ground zero." Rescue workers also used the phrase "The Pile", referring to the pile of rubble that was left after the buildings collapsed.