Already suffering brutally low temperatures, some Chicago residents were alarmed on Wednesday night by booming sounds, reports WGN TV. "I thought I was crazy! I was up all night because I kept hearing it," a local viewer posted on Facebook. Per CNN, meteorologists suspect the sounds were caused by underground water freezing and expanding—a weather phenomenon known as cryoseism, aka a "frost quake." The expanding ice causes soil and rocks to crack, resulting in the booms, explains the Chicago's Daily Herald.
"It's not like an earthquake because it's not the earth moving or something we are able to record," says USGS geophysicist John Bellini. "Compared to an earthquake, it has a relatively small energy source." Others, however, suggest the sounds might have been caused by steel and wood contracting as temperatures fell overnight to minus 20 degrees and lower. C. Kent McKenzie of the Lake County Sheriff's Office, for example, thinks it's more likely the booms came from houses and other buildings. "To me, the popping sounds seems more like structural cooling," he said. (Two kind acts helped Chicago's homeless in the extreme weather.)