What was that?

That was the most-asked question around southeastern Brunswick County on the night of Wednesday, January 15. At about 9:15 p.m., many area residents reported hearing a low rumble accompanied by brief but violent shaking of their homes and other structures. A second, less intense incident occurred a few minutes later.

Reports of the anomaly came from Oak Island, Caswell Beach, St. James and parts of Southport and Boiling Spring Lakes as well as Shallotte Point and Carolina Beach.

Responders logged nine calls to 911 at the county’s communications center, said Emily Flax, spokeswoman for the Brunswick County Sheriff’s Office.

“Yes, our center did receive quite a few calls as is the case when this has happened in the past. Our deputies were dispatched to the areas in county that were called in,” she said. Deputies found nothing unusual, she said.

“I wish I could tell you what it was, but we are definitely not scientists and I wouldn’t want to speculate,” Flax added. “I know it certainly is frightening for people when it happens.”

At 9:51 p.m., the Southport Police Department posted on social media that the department was aware of the incidents, but there was no reason for residents to be concerned.

Longtime residents have experienced similar shakes and noises before but Wednesday’s events seemed out of the ordinary.

Twitter, Facebook and Nextdoor lit up with multiple reports of shaking and, as could be expected, multiple thoughts on what caused them.

Several residents who’ve been here for more than a decade noted that the so-called Seneca guns usually happen during the day and are usually accompanied by a loud booming sound.

Whatever shook structures last Wednesday happened at night and, according to multiple observers, had more of a low rumble than a big boom. So, what was that?

It’s easier to say what it was not.

Local radar and meteorological reports show there were no active thunderstorms or lightning in the region at that time. The U.S. Geological Survey confirmed there was no recordable seismic activity at the time, although there was an earthquake hundreds of miles offshore in the Northern Mid-Atlantic Ridge at about 7 a.m. and another in Dyerburg, Tennessee at about 2:30 p.m. that day.

The U.S. Coast Guard stated there were no known military operations in the area at the time. It is worth noting, however, that the U.S. Navy Carrier Strike Group 4 conducted training exercises off the coast of South Carolina and Georgia the next day, January 16, through this Friday, according to the Federal Aviation Administration (FAA). The FAA issued the release as a warning to pilots and mariners because the exercise involved temporary scrambling or distorting of GPS data.

On the west side of Oak Island, there was no heavy construction going on at night, nor were jet engines audible outdoors shortly before and after the shakes. That would seem to eliminate sonic booms.

So, what was that?

“The real thing is that we just don’t know,” wrote Roger Shew, a professional geologist and lecturer at the University of North Carolina at Wilmington’s Department of Earth and Ocean Science and the Department of Environmental Science.

Shew stated that while noises are often associated with earthquakes, the Seneca guns have never been associated with seismic activity.

“In Italian they are called ‘brontidi,’ i.e. thunder-like; Japanese call them ‘uminari,’ which translates to ‘the rumbling of the sea;’; in Dutch they are ‘mistpoeffers,’” Shew stated. That term means “fog belch,” and it was foggy on Oak Island at the time.

The sound and effect are also called skyquakes and Seneca guns.

In “Rip Van Winkle,” Washington Irvine whimsically suggested the skyquakes were caused by ghosts playing nine-pin in the mountains.

James Fenimore Cooper described them as Seneca guns, for the finger lakes in New York state. He and others have noted that reports of the sounds and shakes go back to native Americans, long before gunpowder was brought to the continent.

Cooper’s story reads in part:

“It is a sound resembling the explosion of a heavy piece of artillery, that can be accounted for by none of the known laws of nature. The report is deep, hollow, distant, and imposing... No satisfactory theory has ever been broached to explain these noises... The most that can be said is, that such sounds are heard, though at long intervals, and that no one as yet has succeeded in ascertaining their cause”

Some of the theories are shallow earthquakes, tsunamis, aircraft breaking the sound barrier and other military operations, quarrying activity (which can create rock bursts when materials are suddenly freed after being trapped for thousands of years), offshore landslides, methane gas release from ocean floor, electromagnetic noise, thunder (rolling), sound traveling further and being amplified by weather inversions (trap sound in air layer and amplify).

Shew said that atmospheric activity seems the most likely explanation, but that theory has never been proven.

“The mystery remains for now,” he said. “But don’t we all love a little mystery?”