A list of some of the nation's iconic structures would be incomplete without mentioning the 110-story Twin Towers and five smaller buildings of the World Trade Center in New York City, which were destroyed by a terrorist attack on September 11, 2001.
The Twin Towers, which were located in lower Manhattan, opened in 1973. Tower One was 1,368 feet tall, and Tower Two was 1,362 feet tall. Of the approximately 50,000 people who worked in the 13.4 million square foot complex, 3,000 died when hijackers slammed two passenger jets into the buildings' upper floors on that fateful day. Construction is underway to rebuild the World Trade Center complex, with an expected completion date in 2012.