But officials aren't sure what it is.
A series of mysterious booms have folks stumped in the small town of Clintonville, Wisconsin.
The noises have been rattling nerves for at least two nights this week.
City Administrator Lisa Kuss says it gets your heart racing because you don't expect it.
The experts have ruled out earthquakes or water pressure problems and there are no military bases in the area.
It was kind of scary at first to 21-year-old Jordan Pfeiler.
He says then it got cool.
Now, Pfeiler says the underground booms are just annoying.
