The mysterious booms heard in Clintonville, Wisconsin, had experts stumped—but there may be a simple explanation. "The mystery is solved. We have experienced an earthquake here in Clintonville," a city administrator explains. Geophysicists call it "swarm of microquakes," reports the Appleton Post-Crescent, and the administrator explains that the fact that Clintonville's soil and granite isn't like what's normally found under towns suffering earthquakes could be a contributing factor as well.
Portable detectors picked up a magnitude 1.5 earthquake—which can only be felt within a five-mile radius—early Tuesday. The devices make their way around the country, and very few are in the state; it was a pure "fluke" that they were in the right place at the right time, adds the city official. But some geophysicists aren't 100% convinced. The data "is kind of going to throw everything into a tizzy," says one, who says the depth of the quakes need to be determined: A mile or two would be an earthquake; something shallower might not. (Read more earthquake stories.)