It started off as a prank by comedy group Improv Everywhere in New York in 2002, bemusing a few embarrassed commuters, but has since grown into a global phenomenon.
Scantily-clad riders took part in cities including New York, London, Milan, Bucharest, Berlin and Mexico City.
The rules of the event, which is in its 14th year, are fairly straightforward: underpants must be worn and, although flashy designs are allowed, organisers prefer those involved to look - at least from the waist-up - as though they are going about their daily lives.
In the inaugural event ago only seven people took part, but now thousands in major capital cities around the globe participate in what organisers call a 'celebration of silliness'.
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