The scientific community has been made aware of a strange crack in the earth in Michigan — and now they think they may have an answer.

The crack was first observed in the forest near Birch Creek in the Upper Peninsula of Michigan about five years ago, and now researchers believe that it is being caused by a limestone bulge, according to a Live Science report.

The crack began with a deep boom north of Menominee that shook nearby homes with the strength of a small earthquake back on Oct. 4, 2010. Just a day later, locals found a deep crack on a narrow ridge measuring 360 feet in length and at a depth of 5 feet. The ridge itself was about 7 feet high and 30 feet wide at the largest point, and trees leaned away from the crack at angles as steep as 14 degrees, a clear a indication that this was a new crack and not one that had been there undiscovered.

Now, researchers are saying that a seismic study indicates the bulge is a pop-up in the upper layers of limestone beneath clay soil. The findings were published in the journal Seismological Research Letters.

Scientists made the observation using a sledgehammer that was slammed into a metal ball on the ground, and then measured the sound waves to get a picture of the soil and rock layers below. They found a sharp buckle in the limestone where the crack was located. It’s an indication that it was a pop-up, which is common in quarries when rock removal released a strain on underlying rocks, causing a big upheaval that can result in cracks like these.

“We wanted to look into the crack because we could not find information in the literature on pop-up structures forming outside specific areas,” said Wayne Pennington, dean of the College of Engineering at Michigan Tech and the leader of the research team, in a statement. “As far as we can tell, this is a one-of-a-kind event; but in case it is not, we wanted the information about it to be available for other investigators.”