ST. LOUIS (AP) - This unusual winter got a little stranger last weekend for some people in Missouri who reported hearing mysterious nighttime noises that sounded like loud cracking or rumbling.
Scientists say they experienced a rare natural phenomenon known as a "frost quake," which happens when moisture in the ground suddenly freezes. If conditions are just right, the soil or bedrock cracks like a brittle frozen pipe.
The season has provided ripe conditions for frost quakes. Temperatures have been frigid, but occasional warm-ups have allowed for thawing. And the temperature swings have sometimes been abrupt.
That was the case Sunday in Missouri, where temperatures in the 40s on Saturday gave way to single-digit readings by Sunday night.
Frost quakes were also reported last month in Canada and in Indiana, Michigan, Ohio and Wisconsin.
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