It wasn't an earthquake that shook the ground in parts of Wisconsin, but it may have felt like it.
Residents in several Wisconsin communities including Fairwater, Markesan, Oakfield, Waupun, Campbellsport, Princeton and Brandon, were rattled earlier this week, after hearing loud cracking sounds that even shook homes.
According to the Fond du Lac Reporter, officers from the Fond du Lac County Sheriff's Office responded to an area in the western part of the county after a reports of the noises. Officers were not able to discover a source.
The culprit of the strange sounds, likely the "Polar Vortex."
The Fond du Lac Reporter says the temperature at the time of the noises was around minus 4. Significantly colder temperatures had blanketed the midwest in recent days and the report says frost depth in the area has increased to about six feet.
WWLP reports freezing ground water in bitter cold temperatures can cause some of the frozen ground or even bedrock to break, giving off energy in an event that is similar to an earthquake.
The phenomenon is known as cryoseism. More commonly they are referred to as a frost quake or an ice quake.
This video from FOX 19 in Cincinnati does a really nice job of explaining frost quakes.
The sound of frost quakes can be quite startling, even to those who understand them. According to the Fond du Lac Reporter, University of Wisconsin-Milwaukee seismologist Brett Ketter experienced a frost quake this week.
He initially thought the pipes had bust in his house and also ran outside in his slippers to see if something had hit his house.