MASON CITY, Iowa - All year long, residents have reported hearing a loud booming sound, mostly at night.
"We hear it often late at night. My husband and I are often sitting around, talking, reading and we'll hear these really extremely loud booms and we'll go 'what was that?'"
Karen Campbell is one of the many Mason City residents KIMT News 3 talked to who say they've been hearing the late night boom. The phenomenon has the River City scratching its head.
According to the Mason City Police Department, they've received many calls about the sound, but when they respond, they can't find the source.
"We’ve often wondered like what's going on behind us? Is there something over at the hospital or is it a train that's not too far away? But we've never really found out what the sounds are," said Campbell.
While there are rail yards for both Union Pacific and Canadian Pacific in Mason City, people think something else is causing the sonic oddity.
Mickie Schussler has also been curious about the random, late-night blasts.
"I called into the police department because it's really at the far west of our house, but it's the northwest of our bedroom in the wall and it's very loud so we thought it was the trains,” she said.
Her theory is it may be caused by the limestone her house is built on. Other residents think it could be cracking caused by freezing water in the soil.
"We think maybe it breathes or shifts a little bit, because we have it and I was glad to hear other people did," said Schussler.
KIMT News 3 checked into seismic activity in Iowa. There have been no earthquakes in the state over the past year.