From their website:
Improv Everywhere causes scenes of chaos and joy in public places. Created in August of 2001 by Charlie Todd, Improv Everywhere has executed over 70 missions involving thousands of undercover agents. The group is based in New York City.
Mission: Frozen Grand Central
Date: February 24, 2007
Target Venue: New York Grand Central Station
Number of Accomplices: 207
Half a million people usually rush through this station everyday. But, that day, things seemed to slow down a bit. What would you do if you experienced something like this? One guy tried nudging another person. Some folks took photos and videos of the 'event'. I loved the applause at the end of the spoof.