'No Pants Subway Ride' is an annual event which began as a prank by a group of New Yorkers in 2002 and has made its way to undergrounds all across the planet over the last decade.
The idea behind the winter wind-up is for trouser-less participants to get on the Metro at different stops and to act completely normal, to the surprise of baffled bystanders.
This year, Spain’s capital will hold its second 'Día sin Pantalones en el Metro', in the hope that security personnel will not interrupt the ‘celebration of silliness’ as they did last year.
Anyone can take part in the event, which will kick off at 5pm in Madrid’s Chamartín metro station, in the north of the city.
The only requirement is not to wear offensive underwear such as "thongs or very tight briefs", organizers say.