Chuck Herron heard the loud thud, then another, and another. It sounded like someone was dropping big snowballs on the roof of his home. As his neighbors in tiny Paris, Mo., huddled around televisions Sunday for the Super Bowl, many were startled by similar strange noises. Some even saw flashes of light and called 911. Scientists say the community experienced a rare natural phenomenon known as a "frost quake," which happens when moisture in the ground suddenly freezes and expands. If conditions are just right, the soil or bedrock breaks like a brittle frozen pipe, generating mysterious noises that range from an earthquake-like rumble to sharp cracking sounds sometimes mistaken for falling trees.
This winter has been ripe for frost quakes, known technically as cryoseism. Temperatures have been frigid, but occasional warm-ups have allowed for thawing. And the temperature swings have sometimes been abrupt. That was the case last weekend in Missouri, where temperatures in the 40s on Saturday gave way to single-digit readings by Sunday night. Missouri isn't alone. Frost quakes were reported last month in Canada and in several other states—Indiana, Michigan, Ohio, Wisconsin. The light flashes reported by some people are believed to come from electrical changes that occur when the freezing compresses rocks. (Read more frost quakes stories.)