World Trade Center site
On the morning of September 11, 2001, Islamic terrorists affiliated with Al-Qaeda hijacked American Airlines Flight 11 and United Airlines Flight 175, both which were en route to Los Angeles, and intentionally crashed them into the two main towers of the World Trade Center. The towers collapsed within two hours of the collisions. 2,606 people, including 2,192 civilians, 71 law enforcement officers, and 343 firefighters, who were in the towers and in the surrounding area died in the attacks, as well as 147 civilians and the 10 hijackers aboard the two airliners. After the collapse of the World Trade Center, hospital workers and law enforcement officers began referring to the World Trade Center site as "Ground Zero".
The Lone Gunmen
One retrospectively relevant aspect of this pilot episode is that the airliner has been hijacked (via remote control of the plane's autopilot) and, by the end, both Byers and his father have boarded the plane to try to stop the hijacking. Through the aid of the other Gunmen, they are able to regain control of the plane and just miss crashing into the World Trade Center with the airliner. This, of course, was before the actual September 11 attacks against the World Trade Center later that year.