They're called "flash mobs," and they started popping up in the U.S. a few years ago. Coordinated via the internet, large groups converge at a particular place, where they do ... something. It almost doesn't matter what; as evidenced by the agendas of a few early flash mobs:
"More than one hundred people converged upon the ninth floor rug department of Macy's department store, gathering around one particular very expensive rug. Anyone approached by a sales assistant was advised to say that the gatherers lived together in a warehouse on the outskirts of New York, that they were shopping for a Love Rug, and that they made all their purchase decisions as a group." "About 200 people flooded the lobby and mezzanine of the Hyatt hotel in synchronized applause for about fifteen seconds."
Public pillow fights have also been arranged, as described right here on this blog. On Wednesday, however, flash mob history was made in London's Victoria Station. Approximately 4,000 "mobbers" jammed the station and, at exactly 6:53pm, whipped out iPods and began dancing to their own music, in what can only be described as a massive, silent and incredibly surreal rave. To get the full effect, check out this video: