Sunday, January 13th, marked the 12th annual "No Pants Subway Ride," aka the day when people drop trou on the subway because...because no pants, that's why. Improv Everywhere, the pranksters behind this long-running event, organized a gathering of 4,000 participants in Brooklyn that quickly spread into Manhattan transit. In addition to major metropolitan areas like Boston and San Francisco, similar pants-less events took place all around the world in places like Mexico City and China.

At this point, New Yorkers are more likely to regard the pantsless riders with a shrug of "Oh, is it mid-January? Must be time for No-Pants Day" than with any sort of genuine shock. Still, you do get some amusing photos of surprised older people. And, hey, underwear!

Check out the best of 'No Pants Subway Ride' below. You can also watch Improv Everywhere's NSFW video of the big day below.