When the planes struck the World Trade Center, air traffic controllers shut down all flights across the country. This videos shows all of the planes traveling through the air right before and after that moment.
The thing that's really insane here is that you can absolutely see that there's one moment when the entire country goes, "Oh, holy crap..." and everything just SLAMS to a halt.
Reddit user iamanis offered an interesting story about travelers that day:
"Notice in the upper right corner of Newfoundland in Canada gets a stream of planes when they're ordered to land. 38 planes were routed to Gander, Canada, a town of about 10,000 people total and home of a large airport that was virtually not in use. So in the matter of hours, this small town had to suddenly accommodate 7,000 people for almost a week until they were allowed to finally get passage to fly again. Hotels filled up, and towns people offered passengers to live in their homes."