MYSTERY BOOMS LEAVE SAN DIEGO GRASPING FOR ANSWERS

Everybody loves a mystery. But we like them solved, too, and so far an answer has been elusive for the Big Border Boom.

It rattled windows and shook doors across a large swath of San Diego and Tijuana late Wednesday afternoon, jangling the nerves of residents who’d experienced a similar noisemaker last month, and one a year earlier.

What was that?

The region joined a list of communities from coast-to-coast that are defined in part by unexplained goings-on. “Mysterious Shaking Rattles San Diego County AGAIN,” the website Strange Sounds trumpeted in a headline this week.

It isn’t always aural. Thirty years ago, thousands of San Diegans were drawn to what some believed was the apparition of a slain girl on a blank billboard in Chula Vista. But unexplained loud, shaking noises are the most common, sometimes falling under the gen

eral term “skyquakes.”

On the East Coast, enigmatic booms are known as “Seneca Guns,” the name drawn from a Seneca Lake in central New York, the setting for a short story, “Lake Guns,” written in 1850 by James Fenimore Cooper.

“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,” he 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.”

After Wednesday’s boom here, the first thought of many people — this being California — was “earthquake.” But the United States Geological Survey said no. Their seismic-activity sensors recorded nothing.

This being San Diego, longtime home to military jets, a lot of folks thought “sonic boom,” too. “That wasn’t one of ours,” said Cmdr. Zachary Harrell, a Navy spokesman, who noted that planes breaking the sound barrier are required to do it far off the coast. The Marines? They didn’t respond to a request for comment. Local defense contractors testing some kind of newfangled weapon? Mum was the word there, too, as it usually is with classified military projects.

In 2012, when a similar boom rattled windows and doors along the local coastline, initial “not us” denials from the military gave way to an admission: The pilots in two Navy F/A-18 aircraft had been showing off for guests aboard the carrier Carl Vinson during a family cruise.

“Those two aircraft went supersonic about 35 miles from the coast,” a Navy spokesman said at the time. “Usually you don’t hear the side booms travel that far. It was kind of surprising to us.”

This time around, Humberto Mendoza Garcilazo, a researcher at the Center of Scientific Research and Higher Education in Ensenada, said supersonic airplanes may have been responsible for the “rumble.” But he also suggested it could have come from the day’s stormy weather and drastic changes in temperature and atmospheric pressure.

Brandt Maxwell, a meteorologist with the National Weather Service in San Diego, was skeptical about ties to the weather. He said there weren’t thunderstorms in the area at the time of the boom, which was about 5 p.m., and “even with a strong cold front, you won’t get that kind of rumbling.”

So, for now, a mystery.

Skyquakes

Explosive booms with no definitive origin have been reported all over the world.

India calls them Barisal Guns; Japan, Uminari. Belgians may have the most fun talking about the phenomenon — “mistpoeffers” — although Filipinos come close: “Retumbos.”

The noises have been attributed to various things, from the familiar (earthquakes, sonic booms) to the out-of-this-world. Some scientists have pointed to bolides, large meteors traveling so fast they explode when they hit the Earth’s atmosphere. Others suggest certain atmospheric conditions amplify the sounds of large ocean waves crashing far offshore.

Last year, seismologists at the University of North Carolina published results of a study of the East Coast skyquakes.

They gathered newspaper reports and other accounts of booms between 2013 and 2015 in North Carolina and compared them to data collected by a network of seismographs and atmospheric sensors that were in the area then.

The results ruled out earthquakes. There was no sign of ground-shaking. So the researchers believe Seneca Guns are “an atmospheric phenomenon.”

The network of sensors picked up unusual signals varying in length from 1 second to 10 seconds that corresponded with the newspaper accounts of mysterious booms. But the researchers didn’t have enough information to pinpoint their precise location or cause.

They said they hope to do more studies in coming years and finally solve that particular mystery. And sometimes even the most durable ones do get solved.

New Yorkers were baffled for several years in the mid-2000s about sweetsmelling air that would occasionally waft over Manhattan. The city created a special team to track what became known as “maple syrup events,” deploying them with special canisters to capture the aromatic evidence. In 2009, they traced the smell to a factory across the Hudson River in North Bergen, N.J. The culprit: seeds from fenugreek, used to make fragrances.

To the factory’s immediate neighbors, this was no mystery. They were familiar with the smell, and its effects. “Ooh, it makes you hungry,” one resident told the New York Times.

Sights, not sounds

Thirty years ago, local residents were drawn by the thousands to a different kind of phenomenon, one for the eyes, not the ears.

In June 1991, Laura Arroyo, a 9-year-old San Ysidro girl, was abducted from her doorstep and murdered. A month later, people started gathering beneath a billboard on Broadway in Chula Vista, gazing up to see a murky image of the slain child that emerged as the sun set.

Not everyone saw the apparition. Some who did attributed it to the play of light and shadow across the white canvas, not to inexplicable forces. Others dismissed the whole thing as nonsense, a hoax.

But upwards of 30,000 people arrived every night for a brief period to look at what believers called “The Miracle on Broadway.” Some said they even saw a second figure, which they thought might be Laura’s killer, peering over her shoulder.

“The human eye has an amazing ability to organize vague images into familiar patterns,” a psychologist told the Union-tribune at the time.

Overwhelmed neighbors living near the billboard asked its owner to turn off the lights illuminating the canvas so the crowds would go away. The crowds came anyway. Eventually a color photo of Laura went up on the billboard, along with a phone number for a hot line to report any tips about the killing, which was solved a couple of years later with the arrest and conviction of a man who lived near the Arroyo family.

The nightly pilgrimage dwindled, leaving some to wonder in the same way others are wondering now about rattling windows and shaking doors.

What was that?