It's No Trousers on the Tube Day.
The annual event was started by Improv Everywhere in New York in 2002 - the idea has since caught on, with cities like Berlin, London and Warsaw taking part this year.
Participants must be willing to take their trousers off on the tube and remain straight faced about it.
Irish cities are not on the list, but that might be something to do with the freezing weather.
You can see more on Improv Everywhere's own site.
Quick thanks to Improv Everywhere for the image, it was taken on a tube in Berlin.
