It started off as a prank that bemused a few embarrassed commuters but has since grown into a global phenomenon.
And despite the threat of snow and bleak midwinter temperatures many hardy thrill seekers braved bracing conditions and took to the London Underground today to take part in the No Trousers Tube Ride 2013.
It was all part of the No Pants Subway Ride's 13th annual celebration of riding the world's underground without trousers, with commuters in London joining their counterparts from dozens of cities around the globe to venture out minus their breeches.
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Flashmob: A group of Tube passengers pretend not to know each other and keep a straight face as they take part in No Trousers on the Tube 2013
Chilly: Commuters wait for a train to come along at Arsenal tube station, but there's one thing missing - their trousers
Barefaced cheek: Two female participants brave the cold weather and take to the London Underground in just their underwear
Mind the gap: A gang of pranksters taking part in the London event make sure they stand behind the yellow line while waiting for their train
The aim is to get on the train without trousers and to keep as straight a face as possible.
Participants behave as if they do not know each other and - other than the lack of trousers - dress appropriately for winter donning hats, scarfs and a warm coat.
The hilarious craze started back in 2002 when a handful of New York pranksters decided to cause a stir on a Subway train.
One participant entered the train in just his underwear for seven consecutive stops and, if asked, were instructed to tell people that they 'just forgot' their trousers.
Acting normally: Those who take part are told to go about their everyday business and pretend not to know each other
Say cheese: Other fully-clothed commuters take pictures of fellow passengers who are just in their pants on a packed Picadilly Line train
Untrussed Down Under: Commuters in Sydney, Australia, also took to the train minus their trousers
Balmy weather: Passengers about to board the Sydney CityRail earlier today in comfortably warm temperatures
Confused: Passengers were bemused by less than fully-dressed commuters at a rail station in Berlin
Colourful: Some dressed in garish underwear as they removed their trousers to take part in the No pants Subway Ride in Berlin today
On the 8th stop, a final participant would enter the carriage selling trousers out of a duffle bag - much to the bemusement of other passengers.
The annual event steadily grew each year with more and more people choosing to take part.
But it nearly hit the buffers in 2006 when the stunt was halted by a New York policeman. Eight passengers were handcuffed in their underwear and taken into police custody.
A judge later threw out the case because it is not illegal to wear just your underwear on the Subway.
Surprise: Passengers on a Shanghai train look startled as one passenger, complete with warm jumper and wooly hat takes off his shoes and trousers
Quizzed: Two passengers are highly amused by the prank while another two men ask the trouser-less traveller what he is up to
Untrussed: Passengers on a Shanghai subway train might have their umbrellas to protect them from the rain, but they are likely to get wet legs without their trousers
Startling: Onlookers watch as two girls chat nonchalantly on a Shanghai train earlier today
The No Pants event went global in 2008 with Adelaide in Australia joining eight American cities in taking part in the stunt.
Last year's event saw tens of thousands of people daring to bare their underwear in 59 cities in 27 countries around the world.
In London alone more than 150 took part.
This year Londoners startled fellow passengers on the Picadilly Line. Unlike freezing London participants, those who took part in New York enjoyed a mild winter day, while those who took off their trousers in Sydney basked in balmy summer weather.
Events also took place in cities such as Sofia, Stockholm, Berlin and Beijing.
Hilarious: A group of men share a laugh as they take part in No Pants Day in Sofia, Bulgaria
Under-dressed on the underground: Fellow passengers smile as two commuters board the train holding their trousers in the Bulgarian capital
Sofia so good: Passengers without pants queue to board a train and provide amusement to fellow travellers on the train in Sofia
Trouser-less: Two girls take off their jeans in Stockholm, Sweden
Undressed to impress: One commuter with a particularly loud pair of pants stands on the platform of a station in Stockholm
