The ground rumbled and residents heard a "boom." It wasn't an earthquake, but likely a rare "frostquake" that American Midwesterners experienced in frozen Wisconsin, say meteorologists. It's a geological phenomenon that occurs when water that drains into the ground freezes, expands and cracks the surrounding frozen earth and rock. Similar events also occurred in Indiana and Pennsylvania.
"It would sound like someone drove into your house," said National Weather Service meteorologist Justin Schultz.
Frostquakes are relatively rare events, Schultz said, but not unheard of in Wisconsin. The shaking is often mistaken for a standard earthquake but usually does little damage. The rumbling was heard in four south-eastern counties in Wisconsin.
No injuries were reported in the wake of the frostquake, which is also known scientifically as a cryoseism.
Some officials speculated that the noise was instead linked to nearby Air Force training flights. But just days earlier Indiana residents reported a similar series of booms that emergency management officials declared "cryoseismic activities or 'frostquakes,'" and said that there was no evidence of any other cause.
Residents of York County in Pennsylvania also reported similar booms, some strong enough to rattle windows, that officials also suspect were due to frostquakes.