Susan Saunders began hearing the explosions at random hours, day and night, in February at her Alhambra home. The first one came at 3 a.m.
"I thought, 'Somebody's blowing things up,'" said Saunders, 60. "It really lifted my windows."
But she heard it again and again. Her neighbors started hearing it. Police were called. They received 114 calls about the noises since mid-February, according to Chris Paulson, the city's administrative services director. He told the Pasadena Star-News local government agencies did not know what the cause of the sound was.
City officials are baffled; one blast interrupted a city council meeting.
"All of a sudden we hear this loud sharp explosion — very quick," Paulson said. "We all flinched and looked around and didn’t see smoke or flames or light."
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They called Caltech seismologists. But they don't have an answer.
"There's nothing seismic that I can see," said Jennifer Andrews, a staff seismologist at Caltech in Pasadena, who was asked by Alhambra city officials to check earthquake data for Feb. 22. "What that phenomenon might be, I don't know. I haven't heard the noises."
The seismograph picks up pressure waves from things like thunder and helicopter sounds. But the Alhambra explosions are baffling.
"Whether it's a man-made or natural phenomenon, I don't know," Andrews said.
She hopes to learn more by the end of the week.
Saunders, a school teacher, said the last time she heard it was in early June.
She suggests that schools set up a science project to have students investigate the source of the booms as part of a class project.
Residents, meanwhile, are speculating online about the source of the sounds. Is it a job for Mulder and Scully?
"I believe that anything's possible, but I don't think these are aliens," Saunders said.
Angie Crouch contributed to this report.