September 11, 2001 stands in history as one of the most tragic days this country has ever known. And nowhere was that tragedy felt more keenly than in New York City, where a senseless act of terrorism claimed the lives of thousands of innocent people and forever altered the city skyline. On that day, all 7 of the original World Trade Center buildings were destroyed, and over 12 million square feet of office space was converted to rubble. Fifteen years later, Ground Zero has undergone a transformation, and vitality and progress honor the memory of those who were taken.