Clintonville - Authorities are flummoxed as to the source of mysterious underground booms that resonated again in the eastern Wisconsin city of Clintonville early Wednesday, but hope a public meeting will calm residents' nerves.

No one has come up with a feasible explanation of the loud disturbances that shook people awake in Clintonville on Sunday and Monday nights, and rattled sleepers again as the sun rose Wednesday.

City administrator Lisa Kuss said the booms have roused people from their beds and into the streets - some in pajamas.

'It startled everyone. They thought something had hit their house or a tree fell on their roof,' Kuss said Wednesday. A police dispatcher took more than 30 calls from concerned residents between 5 a.m. and 7:30 a.m.

City officials scheduled a Wednesday evening meeting at the high school in Clintonville to give residents a chance to vent their frustrations about the disturbances.

'With everything that's been going on, we need to give people a chance to get together,' Kuss said.

Possible explanations for the underground ruckus have been nearly exhausted, she said.

City officials have checked and rechecked methane levels at the local landfill, monitored water, sewer and gas lines, contacted the military about any exercises in the area, reviewed mining explosive permits and inspected the Pigeon River dam next to city hall.

'To me, it just seems like the most logical explanation is some earth-moving, geological thing,' Kuss said. 'But then why is it not happening elsewhere? It's hard to believe my little city is geographically different than the rest of the world.'

Harold Tobin, a University of Wisconsin-Madison seismologist, said there are similar reports of booms elsewhere in the U.S. and farther afield from time to time. Sometimes they're explained, sometimes they're not, he said.

'I'm as intrigued and as puzzled as other people are,' he said Tuesday.

A seismic station near Clintonville has recorded unusual ground shaking since Sunday night. Tobin said such activity can be caused by quarrying, mining and heavy truck traffic, but since the city ruled out those sources - there are no mines or major construction in the area - the university and the Wisconsin Geological and Natural History Survey will likely take a closer look at the station's data.