On the evening of January 30, some Chicagoans were startled by seriously loud noises. It was difficult to pinpoint where they were coming from.
Those noises probably weren't gun shots or fireworks; although it hasn't been confirmed by authorities, it's most likely the people were experiencing a weird phenomenon known as 'frost quakes'.
These booming noises, also known as cryoseisms or ice quakes, are seismic events caused by the sudden freeze and expansion of underground water, which can happen due to a temperature drop.
The expanding ice can split rocks and crack the ground, producing loud noises in the process.
"What we believe is happening is when there is a significant plunge in the air temperature and the saturated soil cools quickly, the ice in the ground can expand rapidly enough to create a loud boom noise at the surface," geologist Steven Battaglia told the Daily Beast.
"Based on the temperature changes in Chicago, it is possible that any areas with saturated ground could have resulted in water expansion and a 'pop' noise near the surface."
Those temperature drops have been blasting the northern US – with wind chill in Chicago clocking in at minus 50 °F (-46 °C) due to the polar-vortex.
And they weren't even the only frost quakes happening this season. According to WHP-TV, other frost quakes were reported in Pennsylvania, and in the Indianapolis area as well, according to WISH-TV.
And these sorts of events could be getting more common.
"With warming temperatures from climate change, it is possible that more frost quakes could occur in the future," said Battaglia
WGN TV first suggested the frost quakes on their social media, and many residents were relieved to have a name for the noise keeping them up.
"I heard one last night. Checked my whole house with a huge knife in my hand!" exclaims one commenter.
"Thanks for posting WGN! I was up all night thinking it was the pipes, roof or the furnace!" said another.
But despite all the commotion, we would totally recommend going and checking out Lake Michigan – the cold has turned it into a strangely surreal winter wonderland.
It might even make you forget about the giant booms of expanding water under the city, and, you know, the whole climate change issue.