Started in 2002 by Improv Everywhere, a prank collective based in New York, "No pants subway ride" started with only seven participants.

12 years later, the day is now marked in over 60 cities around the world.

On "No pants subway ride" day, random passengers take the subway in the middle of winter, without wearing trousers. Participants wear all of the usual winter clothing on their top half such as hats, scarves, and gloves and do not acknowledge each other's similar state of undress.

Some riders enter the New York subway in their underwear as they take part in the 2014 No Pants Subway Ride on 12 January 2014.

A participant takes part in the "No Pants Subway Ride" in Beijing on 12 January 2014.

couple taking part in the "No Pants Subway Ride" remains on a metro train in Buenos Aires on 12 January 2014. "No Pants Subway Ride" is an annual event in which transit passengers ride trains without wearing pants in January. The event is observed in dozens of cities worldwide.

Two riders in the New York City subway in the underwear as they take part in the 2014 No Pants Subway Ride January 12, 2014.

Passengers without pants ride a train during the "No Pants Subway Ride" in Chicago, Illinois on 12 January 2014.

A woman without pants waits on a subway platform during the "No Pants Subway Ride" in Chicago, Illinois, on 12 January 2014. A man looks on at a woman with no pants on, as she appears normal.

Passengers without pants ride a train during the "No Pants Subway Ride" in Chicago, Illinois, on 12 January 2014.

Passengers without their pants exit a subway during the "No Pants Subway Ride" at Alexanderplatz subway station in Berlin on 12 January 2014.