Mysterious Underground Booms in Wisconsin

For
the past few nights, residents of the eastern Wisconsin city of Clintonville
have been woken up by mysterious underground booms.

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

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."

Link

So, armchair sleuths, what could it possibly be? My money’s on the Mongolian
Death Worm.