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."
And these sorts of events could be getting more common.
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.