A gigantic boom has been reported in Alabama, and it’s part of a series of booms heard not just in the United States but around the world that have scientists and authorities scratching their heads. The “Bama Boom” took place in the last week, rattling houses and shaking windows as well as prompting residents to call 911.

Other booms reported in Idaho, Colorado, New Jersey, Detroit, as well as in the Middle East and Australia have further vexed authorities, who have struggled to come up with a good explanation. In all, there have been a total of 64 reports this year.

The Colorado boom had a similar set of circumstances to the Bama Boom. It happened on Nov. 20 at around 9 p.m., with residents reporting a loud boom that shook homes.

“It was just like boom and the trailer shook, and I thought, ‘what the heck was that?’” said Ray Armijo, a resident who has served in the military, according to a Tech Times report. “It kind of scared me a bit.”

The event prompted the local police chief to contact the Air Force and the Federal Aviation Adminsitration to ask if a nearby air base was the cause, but they said it wasn’t them, and the fire department couldn’t figure out where it came from.

Some theories like supersonic aircraft or a meteor exploding in the atmosphere appear to be the most popular explanations, but even those hypotheses have problems. They’re definitely not from earthquakes, the USGS says, and the National Weather Service says it’s not a weather phenomenon.

Bill Cooke, head of NASA’s Meteroid Environment Office at the Marshall Space Flight Center, points to eyewitness accounts of a vapor trail and suggests an exploding meteor is the most likely cause. Indeed, a meteor that exploded over Chelyabinsk, Russia back in 2013 caused a very similar chain of circumstances, with a loud explosion shaking the city and shattering windows. It was captured on many dashcam videos by drivers on their morning commute.