It was a sleepless week for the 4,600 residents of Clintonville, Wis. In an unsettling twist on things going bump in the night, the city has endured since Sunday a series of loud booms whose source is as maddeningly elusive as a phantom itch. The U.S. Geological Survey (USGS) claims that a recent "swarm" of low-grade earthquakes might be the culprit, but even USGS geophysicist Paul Caruso says he's "skeptical" that such small temblors could yield this kind of noise. Here, a guide to Clintonville's "baffling" booms: 

What do they sound like?
Some locals have likened the noise to thunderclaps. "I thought one of my trees fell onto the house," resident Al Miller tells the AP.

How many booms have there been?
A series of five booms hit the town Sunday night, and more were reported on Monday and Wednesday. "There's no warning, it's just 'bam,'" city administrator Lisa Kuss tells the Associated Press. Residents say the eerie noises, which seem to come from underground, are accompanied by tremors that shake windows and floors. 

How have Wisconsinites responded?
First with puzzlement, then with increasing annoyance. Some even packed up their things and left. "Our dog is scared, our neighbors are leaving and stuff, so we decided we are going somewhere else for a while," Dennis Padia tells ABC News. "It's that loud, and it bothers you. You can't go to sleep."

If not earthquakes, what's causing the booms?
Local authorities quickly ruled out drilling activity, clandestine military experiments, and erupting sewer and gas lines. One possible culprit is the warm winter, which could have caused water to leak out of cracks in the granite that undergirds the area, Steve Dutch, a geologist, tells the AP. The newly waterless gaps could cause the granite to shift noisily. The residents have their own theories, from hydraulic fracturing (or fracking) to exploding meth labs. "And the aliens, of course, there's always aliens," says resident Jordan Pfeiler.

Has anything like this ever been heard of before?
Yes. Similarly mysterious booms have recently been reported in California and New York. And it's not just a new phenomenon. A rich lore surrounds such noises, says Whet Moser at Chicago Magazine. Through the centuries, observers have named them brontides, mistpouffers, Seneca Guns, and gouffres. As the writer James Fenimore Cooper recounted in his 1851 story The Lake Gun: "It is a sound resembling the explosion of a heavy piece of artillery, that can be accounted for by none of the known laws of nature."

What about the earthquake theory?
The scientists with the USGS say it's possible that earthquakes could make a booming sound. Caruso says they can "generate seismic energy that moves through rock at thousands of miles per hour, producing a sonic boom when the waves come to the surface." But, with many questions still unanswered, the mystery will likely continue.

Sources: ABC NewsAssociated Press (2) (3), BoingBoing, Chicago MagazineDiscovery News, The Washington Post