The Flogsta Scream occurs every night at 10 p.m. when university students living in Uppsala’s Flogsta neighborhood stop what they’re doing and let out a collective scream from their windows, balconies, and rooftops.

It’s a tradition that dates back to the 1970s, though details regarding how or why the tradition started are difficult to come by. The most extensive account of the Flogsta Scream comes from Wikipedia, which postulates that the Flogsta Scream may have started as a way to blow off steam during exam periods or possibly to commemorate a student who committed suicide. The article, however, cites no sources and additional information is scarce.

Still, it’s a useful thing to know, should you find yourself on a nighttime stroll in Uppsala, that the spontaneous shrieks from the nearby dormitories are no cause for alarm.