What is that noise? It could be a boom from a 'frost quake.'

Boom.

When the noise echoes through the room, it sounds like a branch from a tree landed on the roof or maybe an ice chunk dropped from an airplane.

What is it?

Meteorologists said Wednesday that sound -- and other "cracking" sounds inside and outside houses -- is called a cryoseism or a "frost quake."

Others say it's the sound made by wood and steel expanding and contracting under extreme stress from temperatures dipping past minus 20 degrees overnight.

Several media agencies have reported the booming noises could be attributed to cryoseism, which occurs when underground water freezes after temperatures rapidly drop during harsh winter weather.

The freezing water puts stress on the soil, which causes the soil to crack under intense pressure, scientists report.

That cracking soil creates a loud "boom."

John Bellini, a geophysicist from the United States Geological Survey, said frost quakes are an "interesting oddity." He said the booms are not the same as standard earthquakes and do not register on seismographs.

"It's not like an earthquake because it's not the earth moving or something we are able to record," Bellini said. "Compared to an earthquake, it has a relatively small energy source."

However, C. Kent McKenzie, the 9-1-1 director of the Lake County sheriff's office, said most of the noises are due to the thermal expansion and contraction of houses under the intense pressure from a severe temperature drop.

McKenzie, who studied geology in college and once served as the emergency management director for Lake County, said cryoseism requires liquid in the days before frost quake occurs. The ground here has been frozen for more than two weeks before the latest onslaught of extreme cold, he said.

"To me, the popping sounds seems more like structural cooling," McKenzie said. "When all that metal and wood expands, then shrinks from the cold, weird things can happen."

For the most part, McKenzie said, the popping noises are nothing to worry about. However, he said, in extreme circumstances, people should call for assistance.

"We have not received any calls at the 9-1-1 center from something like this, so the noises are generally nothing to be worried about," he said. "But should something happen, people should not hesitate to call for help. Most fire departments are willing to come out and check things out to make sure there are no problems."