If you hear a loud crack or boom, it may be because of a frost quake

While you enjoy a long winter’s nap as brutally cold temperatures grip much of the country, you may awaken to a loud crack or snap. That scary sound could be a frost quake — also known as a cryoseism — and it happens when rain and ice seep into the soil, then freeze and expand.

A frost quake can cause the ground to shift and feel like an earthquake, although they are actually unrelated occurrences since earthquakes happen when a large piece of the earth’s crust moves. That being said, “frost quakes” can still be detected by seismic wave monitors, according to the Smithsonian.

The resulting noise is enough to wake you up out of a deep sleep. Toledo, Ohio, meteorologist Ryan Wichman took to Twitter to say he heard one on Jan. 29:

“Just heard my first ‘frost quake’ of the night,” he wrote. “That’ll wake you up in a hurry!”

Others, like Twitter user Sam Johnson, also reported hearing loud bangs that were later attributed to frost quakes.

What causes frost quakes?

In order for a cryoseism to happen, there has to be a perfect storm of conditions in place, so to speak. First, the ground must be saturated with water, according to WGN. Next, the temperature must drop rapidly in a short time period for the water to freeze and expand. It’s that expansion that causes the soil and rocks to make a “bang” or “boom” sound.

Check out WGN meteorologist  Morgan Kolkmeyer’s explainer GIF on Twitter:

Sometimes the quakes can also produce visual evidence, as was the case in Connecticut. NBC Connecticut meteorologist Ryan Hanrahan tweeted out these photos of large cracks in the ice covering the ground:

“More than a thousand viewers told us they heard the frost quakes Sunday night and now we are getting some pictures of the giant cracks that developed in the ice across the state,” he wrote.

Frost quakes are not the only unusual weather phenomena produced by the record-breaking cold. From “pancake ice” in Chicago to “snow waves” in upstate New York to partially frozen Niagara Falls, winter sure is leaving its mark this year.