Scientists are looking into a series of low-magnitude earthquakes that have hit a small Wisconsin town recently. The mysterious part is, geologists say that quakes of such low magnitude aren’t generally felt by anyone and are only recorded on seismographs; yet residents of Clintonville have been reporting disturbances–including loud booming noises spaced several minutes apart and shaking of the ground–since last Sunday.

Just after midnight on Tuesday, a 1.5 magnitude trembler hit Clintonville, which is about 40 miles west of Green Bay, and when calls of a disturbance began pouring in, geologists went through seismograph reports to find that earthquakes did occur, but some scientists are skeptical that such a low-magnitude event would generate enough energy to cause loud noises. However, officials have ruled out the sounds as anything man-made. Steve Dutch, a geologist at the University of Wisconsin-Green Bay, said a 1.5 magnitude earthquake produces the energy equivalent of 100 pounds of explosives and could produce loud sounds.

But he was reluctant to describe Tuesday’s event as an earthquake, saying the term is “generally used to refer to widespread stress in the earth’s crust”. He speculates that what happened in Wisconsin could be groundwater movement near the surface, or the thermal expansion of underground pipes.

Still, Dutch can’t rule out an earthquake entirely. There are many different types of rock found in the Wisconsin landscape–including quartzite, sandstone, limestone, and calcite–and are thought to be excellent transmitters for seismic energy. This could explain the loud booms heard by residents.

“If you’ve got something causing a little bit of shifting underground, it may take a while for whatever is causing it to play itself out,” he said.

Tuesday’s quake registered on seismographs as far away as central Iowa, which is strange for such a small trembler.

No damage or injuries have been reported.