As a deep freeze enveloped much of the Midwest on Wednesday, some people in the Chicago area reported hearing loud booms or banging sounds overnight.
That sound possibly could have come from cryoseisms, or frost quakes, according to KTLA sister station WGN in Chicago.
So, here's how a frost quake happens:
First, the ground has to be saturated with water -- like it is in the Chicago-area right currently with all the snow being dumped in the region. When there's a sudden drop in temperature, the water freezes and expands.
The expansion and pressure build up causes stress on the frozen soil and rocks around it, and creates a boom noise, according to the station.
— Morgan Kolkmeyer (@MorganKolkmeyer) January 30, 2019
WGN posted about frost quakes on social media, and numerous viewers commented about hearing them.
"I thought I was crazy! I was up all night because I kept hearing it," one wrote on Facebook. "I was scared and thought it was the furnace. I kept walking through the house. I had everyone’s jackets on the table in case we had to run out of here."
Another viewer commented: "I was up all night thinking it was the pipes, roof or the furnace!"