Out of nowhere on Tuesday night, around 8 PM, Alex Arevalos, a student and graphic designer in Alhambra, California, ten miles east of downtown Los Angeles, heard a single, loud thud.
He immediately asked his sister if she'd slapped his bedroom wall. "She said she didn't, so I automatically blamed the train," Arevalos, who lives near train tracks, told VICE.
Then on Thursday around midnight, two similar sounds woke up Arevalos's father, and when father and son spoke about it in the morning, the younger Arevalos became convinced it was something abnormal. "This time as soon as I heard it, and heard the walls shake a bit, I listened for the train, but didn't hear anything," adding, "I can tell the difference now [between] the train and the booms."
Arevalos is far from alone. According to the local news site Alhambra Source, residents first reported hearing the booms on February 16 when a woman named Noelle Dominguez alerted her neighbors to them in a private section of the community social network nextdoor.com. "I know this sounds weird. But since [I've been] living in Alhambra, every other night or so I hear a loud explosion-like noise," she wrote. Soon, other nextdoor.com users shared similar experiences with the booms, according to Alhambra Source.
Two nights later, Alhambra Police Department posted about the booms on Facebook. Just after 8 PM, officers received reports of "a loud explosion heard in the northern end of our city." The police wrote that they've received multiple similar reports in recent weeks, but that "unfortunately, we were unable to locate the origin."
"We are as puzzled as everyone," Jerry Johnson, the Alhambra police sergeant, told VICE. He said two on-duty officers heard the booms recently, and they rushed toward the source, arriving just 90 seconds after the sound dissipated.
"And then nothing," Johnson said.
In the comments of an Alhambra PD Facebook post, one Facebook user named Anthony Ruiz called the booms, "much too loud to be a firework." Another user named Christopher Keller described them as akin to a sonic boom, saying he felt a "pressure wave." But he added that they were too close to be sonic booms. An isolated series of sonic booms shook New Jersey in late January—but that was an isolated incident brought on by several fighter jets breaking the sound barrier around the same time above the area.
Chris Paulson, the administrative services director for the city of Alhambra, also called it a "sonic boom type of sound," made all the more strange by the fact that it's being reported across an unusually wide area. "We've investigated, and it's probably about a mile north to south," he told VICE.
According to Alhambra Source, there are construction projects going on in the area, but the local public works department "does not believe that the projects are the source of the noises." According to Paulson, that's because, "there's simply no construction going on when those noises are heard."
VICE contacted a municipal consulting company called Transtech, an engineering firm that contracts for Alhambra, inspecting safety concerns at city construction projects. Transtech's Alhambra city building official, Ayla Jefferson, told us she had heard of the booms, but has "no knowledge" of their origin.
Meanwhile, the booms continue unabated. For Arevalo, they've become part of life in Alhambra. He described the most recent explosions he heard as "just kind of there." Since he's been living near a train for 13 years, he says he's become accustomed to noise in general, adding that "the only thing affecting my sleep is school."
But not everyone is tuning out the booms. According to Sergeant Johnson, "We're getting calls on this two or three times a day."
"It is a mystery," said Paulson.