Middlebury - A quiet summer afternoon was shattered around 3:00 Monday, when a loud boom could be heard and felt for miles around.

"It was very loud," Bob Bergstrom said.

"It was impressive that the whole building shook," Netaka White said.

The loud boom was followed by the sound of people asking -- what was that?

"Really nobody knew the source of this," Middlebury Police Chief Tom Hanley said. The department took numerous calls from worried residents. "Big mystery, but we weren't getting any calls of anything bad happening so that was comforting."

From Middlebury to Monkton, Ferrisburgh to Bridport -- even Moriah, New York -- people reported hearing and feeling something unusual. No one had an answer, but everyone had a theory.

"I guess there was a momentary thought that it was a gas line," White said.

"We heard this huge boom, but we thought it was an explosion from the gravel pit mining," Cindy Fox said.

"We figured it must be a meteor or something going fast to break the sound barrier," Hanley said. "We couldn't find any rational explanation for this."

Turns out it was just what it felt like -- a sonic boom. The Vermont Air National Guard said an F-16 broke the sound barrier.

"The aircraft was doing a mach run, which means he broke the sound barrier," explained Lt. Col. Lloyd Goodrow. "He was exceeding the speed of sound and that's what makes the blast you heard."

The pilot was in the sky over Vermont and New York doing what's called a functional check flight. The Air Guard does them about three times a year after major maintenance work, but Goodrow said it is unusual anyone on the ground would hear it.

"We always do these at 30,000 feet. In this case, our pilot was at 40,000 feet. To be honest, he was a little surprised there was any, any knowledge of it down below."

No explosion, no meteorite, not even construction -- just a lesson in the science of sound.

"People get concerned when they hear a loud boom," Chief Hanley said, "so at least we have an answer now, so we're happy."