A series of "loud booming noises" that rattled windows in southeastern Wisconsin Sunday was possibly caused by "frost quakes."
The Muskego Police Department posted on its Facebook page that it had received several reports of the booms within city limits, with posts on social media suggesting they were also heard across Waukesha, Big Bend, Wind Lake, and Vernon.
"Investigating officers were unable to determine a source of the sounds or any evidence that they originated in Muskego," it added. "As of this time, we have not received any reports of damage or injuries from citizens."
But the explanation for the noises could be meteorological, rather than anything more sinister, according to weather experts.
WISN-TV chief meteorologist Mark Baden suggested that the culprit was was most likely "frost quakes," with the National Weather Service's Chris Stumpf telling the Milwaukee Journal Sentinel the same.
ABC News notes that frost quakes happen when water underground freezes, and then expands, causing the surrounding soil and rock to crack.