Imagine leisurely walking through the middle of Grand Central train station in New York City and witnessing a mob hit.

But this is no ordinary mob — and no ordinary "hit."

A large group of people appears suddenly in the middle of the terminal, freezes in mid-motion and, after several motionless minutes, unfreezes and nonchalantly carries on with the day’s business -- as if nothing unusual happened.

It may sound odd, but a group of more than 200 people pulled off such a stunt in January, part of a phenomenon known as a "flash mob" that often succeeds in confusing people who aren't in on the act.

Such flash mobs form through mass text messages, Facebook networks, e-mails and Web sites. Web sites, such as Flashmobaustin.com, are solely dedicated to this mission, billed as "one spontaneous event at a time."

Although many mobs pull harmless pranks, such as shopping in slow motion at a store, others can get nasty and leave a messy mob scene behind.

In San Francisco, the annualmob public pillow fight last Valentine’s Day left a wet mix of feathers on buildings, gutters and cars. City workers were left to clean up thousands of dollars worth of damage, and city officials are considering new regulations.

Mob "agents" could be required to obtain a permit and pay for usage of the event location before it takes place, threatening the very spontaneity and anarchic flavor that define the mobs. And the organizers may be responsible for providing security and portable toilets for the event.

It often is hard even to locate the organizers of such events, but many pasts "hits" that have occurred across the United States also have become hits on YouTube. Here are some of the most popular videos of flash mobs in action, with recent viewership figures:

1. Frozen Grand Central

Description: Estimated 207 people freeze in motion on cue in Grand Central Terminal in New York City.

Posted: Jan. 31, 2008

Posted By: ImprovEverywhere

Views: 16,027,418

Comment: By user wek33, "That was absolutely genious."

2. No Pants Subway Ride 2009

Description: New Yorkers take to the subway without pants.

Posted: Jan. 13, 2009

Posted By: ImprovEverywhere

Views: 6,861,289

Comment: By user 1000Bomb1000, "So why just pants? Take off all...it's better, no?"

3. Slo-Mo Home Depot

Description: New Yorkers shop in slow motion at a Manhattan Home Depot.

Posted: Aug. 24, 2006

Posted By: ImprovEverywhere

Views: 1,419,687

Comment: By user Baha XD, "I don't know why but things like this make me laugh hysterically Really great :]."

4. University of Florida Bang

Description: A large group of students gather and, with hands in the shape of guns, shout ‘bang!’ and play dead.

Posted: April 12, 2006

Posted By: Veritas 1000

Views: 699,495

Comment: By user followingthecrowd, "This is like the best flash mob to do......im doing it in my school later XD."

5. Ninja Flash Mob UC Berkeley

Description: Students break out into ninja warfare in front of Wheeler Auditorium at the University of California at Berkeley.

Posted: Jan. 25, 2007

Posted By: SJavdani

Views: 336,978

Comment: By user schnucklpu, "That’s the greatest thing I’ve ever seen!!! :)"

6. UNC Chapel Hill Rave

Description: University of North Carolina students decide to break up the monotony of final exam week with a "rave" at the undergraduate library.

Posted: Dec. 10, 2008

Posted By: cackalacky789

Views: 283,014

Comment: By user lhaller2, "Loved the look on the librarian's faces ‘How can we shush this?’"

7. Where’s Waldo Flash Mob

Description: Waldo impersonators gather at the Ferry Building in San Francisco.

Posted: Nov. 11, 2006

Posted By: sfmallon

Views: 163,379

Comment: By user n8iveidiot13, "I found waldo... BUT WHICH one... IS THE REAL ONE!?"

8. Texas A&M Flash Mob: Zombies

Description: A&M students acting like crazed zombies roam the school’s halls and walkways.

Posted: Oct. 31, 2006

Posted By: gdiddy8800

Views: 151,282

Comments: By user PsychoFlunky, "That was epic."

9. Flash Mob "Worshiping the Purdue Engineering Fountain"

Description: Students at Purdue University worship the Engineering Fountain with chanting.

Posted: Sept. 25, 2007

Posted By: boilermeiker

Views: 102,303

Comment: By user Skandranon2148, "This is my favorite of all the flashmobs I've seen, including the ImprovEverywhere ones. It's just hilarious. Great job! "

10. San Francisco Pillow Fight 2009

Description: The annual pillow fight on Valentine’s Day in San Francisco.

Posted: Feb. 14, 2009

Posted By: nsarilakis

Views: 4,169

Comments: By user ergonomix, "Man I had sooooooooo much fun!!!"