Clintonville - There's one thing everyone here agrees: It was really noisy.

Exactly what it sounded like is where Clintonville residents differ.

Jordan Pfeiler thought it sounded like fireworks exploding underwater.

Fay Mavis' 27-year-old grandson thought it sounded like a 90-ton wrecking ball slamming into something, while Mavis said it sounded like huge chunks of ice falling from her roof. But then she realized in this heat, ice chunks are long gone.

Marge Bickford thought somebody drove a car into the front of her home.

Beverly Demitriou thought a critter had sneaked into her basement and was causing bedlam.

City Administrator Lisa Kuss said it sounded like a jackhammer.

Cats and dogs didn't know exactly what it was, of course, but they howled and meowed anyway.

A series of loud bangs and booms woke up folks on the northeast side of this Waupaca County community shortly before 2 a.m. Monday and continued off and on until at least 8 p.m. The rattling, shaking, vibrations and noises induced people to pick up their phones and dial 911 as Clintonville's police dispatch center lighted up with more than 150 calls.

The source of the racket? No one knows. Though they're pretty sure what it wasn't.

Earthquake? No. Methane gas? No. Wastewater? Nada. Change in water pressure? Nope. Mining, military missions or dynamiting in the area? Zip.

"All of that came up negative," Kuss said Monday afternoon at City Hall as a Pizza Hut delivery person dropped off pizzas.

It had been a long night and day for city officials as they tried to figure out the source of the loud mystery.

"And it may remain a mystery," said Andrew Carlin, Waupaca County emergency management director. "We're not giving up. We're to the point we're ruling out the easy stuff. So now we're checking with scientists."

University of Wisconsin-Madison geologists said the noises and vibrations were not typical of earthquake activity. However, a seismic station in Shiocton, about 15 miles southeast of Clintonville, recorded unusual ground shaking in the area.

In search of explanation

David Hill, a seismologist and scientist emeritus for the U.S. Geological Survey in Menlo Park, Calif., has done research on mysterious booms and authored a paper on the subject. Reached by phone Monday afternoon, Hill wondered if it was ice breaking up on nearby lakes. But when told winter had left Wisconsin weeks ago, he admitted he was at a loss for an explanation.

"In North Carolina there are sand dunes and there's such a thing as booming sands," Hill said of recent reports of similar-sounding noises in that state. "Of course, booming sounds travel a long ways. Sometimes thunderstorms can be beyond the horizon, but it's not likely that's the source of the noises in Wisconsin at this time of year."

Shari Pfeiler had heard the loud noises for several weeks, but the bangs were only occasional, and since she and her family live close to the Seagrave Fire Apparatus plant, where fire engines and pumping equipment are manufactured, she didn't think much of it.

"Then around 3:30 a.m. all hell broke loose. The booms were coming one right after another," said Shari Pfeiler as she sat on her front stoop next to her daughter Jordan. "The cats were freaking out."

The Pfeiler family ventured outside and saw lights blinking on at their neighbors' houses down the street. Pulling robes tight against the night chill, many came outside and asked each other the same question: What is that?

Mavis looked out her windows and couldn't see anything. Then when the commotion seemed to dial up to Volume 11, Mavis' two grandchildren ran downstairs. When she realized there was no ice to fall from her roof, Mavis wondered if someone's furnace had blown up. It wasn't just her grandchildren who were unnerved.

"She was under the covers," Mavis said as she walked Missy, her black Shih Tzu-Yorkie pup.

Across the street, Bickford's dogs Riley and Rufus barked incessantly as Bickford wondered if some drunk had plowed a car into her house. Then the noises got louder and closer together.

"It was louder at times than a tornado," said Bickford, who admitted she's never been in a twister but heard they sound like freight trains. "When you don't know what it is, it gets kind of scary. If it was thundering and lightning, I could understand."

Demitriou was jolted out of bed like the rest of her neighbors. As her cat, Jingle, clung to her side, Demitriou wondered what kind of wild animal - raccoons? squirrels? - could make such a hubbub in her basement.

"It kept me awake, and I began to wonder what it was," said Demitriou, who by 1 p.m. Monday had been awake for about 11 hours. "People said we had such a warm spring that maybe something was happening underground."

In a lull in the booming and rumblings Monday afternoon, Demitriou said she planned to take a nap to make up for some of her lost shut-eye.