Remembering September 11 (as we do in the United States), I've been mesmerized by this 23 second video showing the halt of air traffic in the USA on that day in 2001.
This animation was created by NASA using FAA air traffic control data from September 11, 2001. It shows the rapid grounding of air traffic across the US, and redirection of incoming international traffic, in response to the 9/11 terrorist attacks. Time is at lower left, number of planes in the air lower right. At 9:06am, FAA issued a ground stop to all traffic not yet departed that would encounter NY airspace ["tier one"- NY, DC, Boston, Cleveland] . A series of rapid decisions followed, including redirecting inbound traffic away from NY and warning airplanes in the air of potential cockpit intrusion. At 9:45am, FAA Command Center decided to close all US airspace for the first time in history. Within a few hours, all commercial air traffic was grounded. This animation is displayed in the National Air and Space Museum's "America by Air" exhibition.
It was the day our skies were silent for the first time in many decades.