Over 3,500 people marched along Manhattan's Hudson River recently, converging from two separate starting points on Nelson Rockefeller Park. And then? Then they had a massive glowstick dance party. That's right: it's another Improv Everywhere MP3 Experiment.

How did the merry pranksters spice things up for this year's revelry? The experiment took late in the day, for one, meaning it ended in an explosion of light and cheers long after the sun went down. It was also the biggest MP3 experiment yet, bringing 3,500 strangers together in slinky syncopation.

Head over to the IE website for more details on how it all went down—and how to get involved next time. And here's our roundup of the eight geekiest stunts the group ever pulled off. [Improv Everywhere]