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SPEARFISH — The mystery of what caused Monday’s loud boom remains.

Some theories have been refuted, while more mysterious references have appeared.

Shortly before 2 p.m., the boom was heard throughout the Black Hills. Some people said it shook their homes or businesses, rattling windows, and scaring them in several instances.

But the noise was heard in a much larger area than the Black Hills. Responses to Tuesday’s Black Hills Pioneer story reported hearing the noise from Western Nebraska to Southeast Montana.

Kathy Griesse reported hearing the noise near the Agate Fossil Beds National Monument south of Harrison, Neb. She said it sounded like the noise came from the north and west of her. Additionally, she talked to people in Crawford, Neb., where people told her windows rattled at the sound of the boom; people in Whitney, Neb., also heard the noise.

On the northern end of reports, Lane Pilster said he heard the boom at his ranch, 14 miles west of Alzada, Mont.

This is about a 200-mile straight-line distance between the two reported locations.

Pilster reported that he and his dad both heard the noise to the south of them.

“The beginning of it was intense, but then faded off with a dull rumbling like a jet was flying by. The sound probably lasted about 8-10 seconds,” Pilster said.

He also said he felt a moderate vibration around 5:30 a.m. Monday, and that it lasted 15-20 seconds.

He wasn’t the only one to hear a strange noise apart from the 2 p.m. event.

Brad Scott, of Spearfish, heard a loud boom in downtown Spearfish around 7:30 a.m. Sunday

He described it as the “sound of about 8 shotguns going off at once.”

So what was the noise?

Some people thought it was an earthquake, or some pressure release from the ground. However, the U.S. Geological Survey’s earthquake monitoring site did not record a tremor of any magnitude in the Dakotas, Nebraska, or Eastern Montana within the last week.

Some people thought it was a sonic boom from an airplane. However, officials with the Federal Aviation Administration and Ellsworth and Minot Air Force bases said it was neither civilian nor military planes from their bases.

OK, so how about a meteor breaking the sound barrier as it fell from space?

Tom Durkin, the deputy director of the South Dakota Space Consortium said he checked with the American Meteor Society and no meteor sightings around 2 p.m. were reported. He did say that it is possible that it was a meteor, but that no one noticed it since it was in the middle of the day.

Other reports came in of the 2 p.m. boom ranging from Interstate 90’s Exit 2, throughout Spearfish, Lead, Deadwood, Piedmont, north of Belle Fourche, to Angostura Reservoir in the Southern Black Hills. People said it sounded as if a vehicle hit their home or office, or a tree fell on their house.

We checked with A-1 Drilling and Blasting, of Deadwood, that was doing work around Lead.

Shawn Stiles, owner of the company, said they did not blast at all on Monday.

He also said that his company, and others that blast, are required to stay below designated thresholds for both vibration and noise caused by explosions. His crews monitor both. And, should someone have a complaint about the noise, he takes it seriously and wants to hear about the complaint.

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