Meteorologists say that water rapidly freezing in the soil amid bitterly cold temperatures can make a startling noise and even cause small vibrations.

These noises are known as frost quakes or cryoseisms, a cold-weather phenomenon that occurs when rain or melted snow in the ground quickly freezes, expanding as it solidifies, when temperatures rapidly fall below freezing. As the ice expands, pressure builds around the surrounding soil, causing it to crack and make booming sounds and light shaking.

Evan Webb, a meteorologist with the National Weather Service in Louisville, Kentucky, said the office has received reports of “loud booms” that are associated with cryoseisms during bitter cold weather.

“I think some people initially wonder if it’s an exploding tree or something,” he said.

Webb said frost quakes are “relatively rare, especially in Kentucky … we don’t get quite cold enough very often to have saturated soil in the winter time to be able to freeze that quickly.”

Webb said frost quakes are “mostly harmless” and single-digit temperatures with a wind chill are a bigger concern.

The weather service office notes in a social media post that the noises can be startling, especially when they happen at night, but they’re generally not a cause for concern.

“Those loud booms aren’t paranormal—they’re cryoseisms (Frost Quakes)!” the post reads.

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