It's startling residents, making houses shake, and causing the 911 phone lines to light up as they did on Monday and Thursday.

There's no doubt folks that live along South Ruth and South Cedar are hearing something.

They made 9-1-1 calls.

But there hasn't been an earthquake, a sonic boom, or any kind of accident near there.

As to what it is, it's as clear as mud.

The neighborhood between Maryville high school and the middle school is a quiet neighborhood, usually.

But Monday morning and Thursday night, residents there heard explosions captured by 911 phone calls.

"Caller: There was a huge boom and my whole house just shook!"

"Caller: We keep on hearing, our houses are shaking up here. We don't know what's going on!"

"Caller: We're hearing and feeling, like, major explosions!

911 Dispatcher: Yes, ma'am. We're had several calls on it. We're trying to figure out where it's coming from."

Residents in the area say it was no small event.

"A violent boom, like a car running into your house," says David Headrick, whose wife made a 911 call.

"A big boom, just like dynamite went off," says Charlie Haun, who thought something blew up in his basement.

"The floor of the living room, which is like made out of wood, you could feel it vibrate through the floor," says Dr. Susan Smith, who heard both Monday and Thursday's explosions.

Emergency vehicles responded quickly to the South Ruth, South Cedar Street area.

And it was not what they found that was so shocking, but what they didn't find.

Maryville Police Chief Tony Crisp says, "We dispatched officers and did a diligent search of that area. Could not find anything that was damaged, nor could we determine the origin of where that explosion came from."

After it was over, there was no devastation, just a quiet neighborhood again.

"So what do you think it was? You tell me! I don't know," says resident John Patterson.

Headrick says, "It's a mystery to me."

Maryville officials say they've checked with utilities, earthquake centers, and they don't know what it was.

"No, really we don't. There again, nothings been damaged," says Chief Crisp.

So residents in that west Maryville neighborhood are again living in peace, for now.

"I think it's just disturbing. I think it's just not knowing what it is, if the earth is going to swallow us up or something like that," says Headrick.

Chief Crisp says they have increased patrols in the area.

Some residents say the explosions have given them cause for concern.

For others, they just hope it doesn't happen again.