What's behind the mysterious, earth-shaking boom of the 'Seneca Guns'?

Enigmatic booming sounds called the "Seneca Guns" have reverberated off parts of coastal North Carolina for more than 150 years, with some powerful enough to rattle windows and vibrate buildings. 

Now, scientists are using seismic data to pinpoint where the explosions come from and what causes them. 

They presented their findings on Dec. 7 at the annual meeting of the American Geophysical Union (AGU), held virtually this year due to the COVID-19 pandemic. But spoiler alert: They haven't quite solved the mystery yet.

Related: 50 interesting facts about Earth 

The name Seneca Guns comes not from North Carolina, but from Lake Seneca in upstate New York where a similar phenomenon occurred. The lake's ominous booming sounds, described in 1850 by the writer James Fenimore Cooper in his short story "The Lake Gun," had at the time been heard for centuries. 

"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," Cooper wrote. "The report is deep, hollow, distant and imposing. The lake seems to be speaking to the surrounding hills, which send back the echoes of its voice in accurate reply. No satisfactory theory has ever been broached to explain these noises."

Coastal North Carolina residents frequently report hearing similar booming noises, with explanations ranging from distant storms or earthquakes, to quarry blasts or even military exercises. To get to the bottom of them, the scientists combed through accounts dating back to 2013, to create a catalog of observations. They then compared those incidents to data collected by the EarthScope Transportable Array, a network of 400 atmospheric sensors and seismographs. Launched in 2003, the array migrates between 1,700 locations in the continental U.S., and the highly localized seismic data that it collects is freely available to the public. Currently located in Alaska, it was installed in North Carolina between 2013 and 2015.

"We wanted to go through local news articles, create a catalog of instances of the Seneca Guns, and then try to verify them with actual seismo-acoustic data,", said researcher Eli Bird, an undergraduate studying geological sciences at the University of North Carolina at Chapel Hill.

Though the Seneca Guns can cause ground shaking, the scientists didn't find any earthquake records that coincided with the events, effectively ruling out ground shaking as the cause of these booms.

"Generally speaking, we believe this is an atmospheric phenomenon — we don't think it's coming from seismic activity, we're assuming it's propagating through the atmosphere rather than the ground," Bird told Live Science. "The data I've most focused on in this project is infrasound data rather than seismic," Bird said, referring to sound that has a frequency below that of human hearing.

One atmospheric explanation could be bolides — space rocks that are traveling so fast when they hit Earth's atmosphere that they explode. Another possibility could be events that originate in the ocean, such as the crash of very large waves or thunder far offshore — "the atmospheric conditions could be such that that gets amplified in a particular direction, or is primarily affecting this localized area," Bird said.

Signals associated with booming varied in length from about 1 second to nearly 10 seconds, with the station near Cape Fear picking up the most prominent signals. Anecdotally, the Cape Fear region is also known for having numerous Seneca Gun incidents. However, the sensor array wasn't dense enough to pinpoint where the signals were coming from, and more data will be required to trace these big bangs, the researchers wrote.

"Presumably, these are not all the same thing producing the booming sounds," Bird said. Some military planes that fly in the area have broken the sound barrier, so some of the "gun" sounds may, in fact, be sonic booms. And even in those cases, a natural signal could be amplifying them even more, he added. 

With the Cape Fear region identified as the most promising location to keep looking, next steps for solving this puzzle would involve collecting more data over several years, using an array of at least three stations with three microphones on each, to more accurately triangulate where the sounds originate.

"Ideally, a dense array located in an area where the signals are most often detected will allow for a more extensive analysis," the scientists reported at AGU. 

But for now, the booming Seneca Guns remain a mystery.

Originally published on Live Science.