At 3:36 p.m. on Saturday, an inbound Red Line train rolled into Harvard Station, holding a number of people wearing no pants. Others looked on with surprise at the spectacle of boxers, briefs and panties. The half-naked riders were participating in the event “No Pants on January 10, 2008,” hosted by The Boston Society of Spontaneity.

“No Pants,” open to all, began at the Alewife station at 3 p.m. There, a crowd gathered of roughly 100 people, mostly college students from throughout the city, including a number from Harvard.

Pantless participants broke themselves up into informal groups, which were assigned stops at which to undress and exit the train. Riders then waited at the stop and boarded the next red line train to pass, momentarily making a spectacle.

Tellingly, most of the event was confined to Cambridge and Somerville: the pantsless crowd rode from Alewife to Park Street and back, where they redressed.

Some of the participants saw a deeper meaning in the event—at least to some extent.

“No Pants Boston inserts something into the culture, something that’s not usually there,” said Michael A. Baskin ’11, who participated in No Pants 2008, the first year it was held in this city. “It doesn’t have a message, but it gives people a story,”

This event, like all those held by Boston SOS, had no cause. According to organization founder James Cobalt, “No Pants” is a social experiment to see how people will react to the unexpected in a very mundane environment.

“It’s like we said, ‘What would happen if?’” said Cobalt. “Most people just don’t really notice.”

The half-naked riders received more than just looks and giggles from others commuters. Some snapped pictures, while others questioned the omission. Participants were instructed to answer these queries with nonchalant lies.

“I said I was going snowboarding and I had way less wind resistance and better powder without pants,” said Lila Shreiber ’12.

“Pants were totally last season,” Rebecca Cohen ’12 told questioners. “When I got up this morning, I knew that today just wasn’t the day to wear pants.”

According to its Web site, the Boston Society of Spontaneity is a “group of friends” who stage events throughout the city, using a combination of “guerilla theater and outright silliness.”

It is currently on its way to becoming a recognized non-profit. Other recent antics held by the group included its Caroling Choir, which performed Christmas parodies such as “Silent (But Deadly) Night.”

Boston SOS It is an offshoot of New York based Improv Everywhere, best know for staging its “freeze” event in Grand Central Station. According to Cobalt, future plans involve guerilla gardening, or renovating public places under cover of night.

Word about the event was spread largely through its Facebook group and the Boston SOS’s mailing list.

Last year, more extensive media coverage made for a better turnout, though Cobalt said too much participation would have diminished the event, making participants seem less like anomalies and more like the norm.

“I still don’t think that too many people would ruin No Pants,” Cobalt said. “It’s just too silly and too much fun.”

—Staff writer Emily J. Hogan can be reached at ejhogan@fas.harvard.edu.