A group of well-meaning recreation enthusiasts in Seoul inadvertently caused the upper floors of a 39-story building to shake, resulting in a mass evacuation.

Set to the tunes of pop group Snap, 17 middle-aged individuals were in the middle of an intense Tae Bo workout when the upper floors of the TechnoMart high-rise started shaking. According to the Korea JoongAng Daily, the shaking lasted for approximately ten minutes.

To determine what happened, a group of scientists recreated the situation in the gym on the 12th floor of the building. According to Chung Lan of Dankook University, the vibrations caused by the workout were recorded on a vibrometer, and this shaking coincided precisely with the unique vertical vibration cycle of the building. The result was an amplification of the vibration and thus more shaking.

The shaking was felt only in the upper floors, so those below the exercise likely did not experience anything.