On any given workday, up to 50,000 employees
9/11 was not the first terrorist attack on the World Trade Center. A bombing in February of 1993 killed six people.
The company that owned the WTC had scheduled a meeting for 9/11/2001 on Tower 1 to discuss what to do in the event of a terrorist attack, but rescheduled the night before because someone couldn't attend.
There's a 9/11 Memorial in Israel constructed of steel from the remains of the WTC.
On 9/11, four people in the South Tower of the WTC managed to escape from a floor above the plane's impact.
Only 1600 victims were identified of the 2749 present when the Twin Towers collapsed on 9/11.
NASA recovered metal from the WTC and used it on Martian rovers as a tribute to those fallen on the 9/11 attacks.