No, you weren't having a bad dream: people were really in their underwear yesterday. It was the annual No Pants Subway Ride Day, a tradition started more than a decade ago in New York by the avant-garde theater group Improv Everwhere that's spread worldwide.

WGN reports that organizers of Chicago’s No Pants ride advised participants to wear “granny panties, boxer briefs, and boy cut underwear.” Let's hope no one was feeling the chilling effects of the Polar Vortex.

Check out photos from No Pants Subway Ride Day below