SAN DIEGO (Tribune News Service) — After another window-rattling boom in San Diego County, the third since February, a pattern may be emerging, if not a definitive answer.

On all three days, U.S. Marines at Camp Pendleton were training with "high explosive munitions," lobbing artillery and mortar shells downrange.

"Depending on atmospheric conditions, the sound of the explosions may be amplified and heard up to 50 miles away," Marine Capt. David Mancilla said Wednesday. "Although we cannot account for every loud noise heard in Southern California, what you are hearing may be a result of our training."

Or maybe not. The training happens year-round — in May alone, there were 14 days of what the Marines call "noise generating events." Shells explode at various times throughout the waking hours, from 6 a.m. to midnight. But widely reported booms don't happen that often.

The newest one, at about 8:20 p.m. Tuesday, rattled doors and jangled nerves as far south as Tijuana and as far east as El Cajon. It was felt along the coast and through a wide swath of central San Diego.

That set off the now-familiar "What was that?" activity on social media, with speculation aimed at the usual suspects.

Earthquake? The seismic sensors at the United States Geological Survey said no.

Sonic boom? People on Twitter cited a flight-tracking website that they said showed planes flying fast enough to shake closed doors. But whose planes? Spokespersons for Navy and Marine wings in San Diego said their aircraft had nothing to do with the noise.

"It's a bit of a mystery to us, too," said Navy CDR Zachary Harrell.

Unusual booms happen all over the world and are known generally as "skyquakes." They've been around for hundreds of years, with explanations offered that run from the mundane to the supernatural.

Scientists sometimes point to bolides, large meteors that explode when they hit the Earth's atmosphere.

Seismologists studying East Coast rumblers called "Seneca Guns" have attributed them to "an atmospheric phenomenon."

In the 1970s, the Naval Research Laboratory in Washington, D.C. examined a string of "startling acoustic events" in New Jersey and South Carolina and ruled out things like meteorites, winter lightning and nuclear detonations. Investigators found sonic booms from "high-performance military aircraft" to be more likely.

Noise sometimes travels in unexpected ways. In 2012, after a loud boom was widely felt along the local coast, the Navy initially denied responsibility but then admitted that two jets had gone supersonic about 35 miles off shore while showing off for guests aboard an aircraft carrier.

"Usually you don't hear the side booms travel that far," a Navy spokesperson said at the time.

High winds and other atmospheric conditions can cause sound to travel unusually far, and that may be in play at Camp Pendleton, which covers some 125,000 acres of terrain and is the Marines' largest training facility on the West Coast. Marines have been training there since 1942.

They blow things up often enough to have a "noise advisory" section on the base website that lists when munitions practice could be especially loud. They publicize a phone number for people to call (760-725-0357) if they have questions or complaints about what they might be hearing.

"The local communities that neighbor our installations are vital to the continued success of our military personnel and we value their feedback," said Mancilla.

The website also has a 90-second public service video announcement that acknowledges the "inconvenience" of the noise while highlighting the economic clout — millions of dollars in civilian salaries and defense contracts — the base wields in the community.

"We do what we can to minimize our impact," it says.

Tuesday's boom followed similar jolts on March 10 and Feb. 16. They've become common enough that some people are having fun with them now. Golfer Phil Mickelson tweeted: "My bad. I was testing a few drivers."

San Diego Mayor Todd Gloria went right to short-hand. "Yes, I heard it," he tweeted. "No, I don't know what it was."

And then he used the platform for a public-service announcement of his own: "Get vaccinated if you haven't yet."

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