An "unknown event," as the U.S. Geological Survey is referring to it, had many San Diegans posting on social media Tuesday saying they had heard a boom and felt sudden shaking.
Whatever it was, it was noticed at around 5:10 p.m., with many who reported the event concentrated in the southern half of San Diego county.
One Chula Vista resident told NBC 7 she heard and felt the vibration of the boom twice, about 10 seconds apart, and described the event as being "like thunder, but without thunder." She added that it "felt like something fell on my roof," although there was "no shaking of our chandeliers, like with an earthquake."
John McLees, also a Chula Vista resident, told NBC 7 he instantly recognized what he heard as a sonic boom.
Meanwhile, Tomas Urtasun was puzzled when the back sliding doors to his South Park home shook loudly, as if something slammed into them, he told NBC 7.
In Clairemont, James Albert was out for a walk when he heard a "very low rumble that lasted about a second." Albert said it didn't sound like an earthquake but, rather, the sound of a large impact.
Other San Diegans reported similar experiences, everywhere from El Cajon to Jamul to Rancho San Diego to Del Cerro to Ocean Beach to Banker's Hill. There were even some reports from as far north as Carlsbad.
Many took to social media to ask others if they, too, had heard or felt the event.
Point Loma resident Sarah Dorsey, who was in her house watching TV at the time, said she felt it like a large truck passing by, with her windows rattling twice, about 5 to 10 seconds apart.
NBC 7 reached out to Navy and Marine officials in San Diego County but was unable to get confirmation regarding any operations at the time that may have been to blame for the event.
A spokeswoman for Marine Corps Base Camp Pendleton confirmed the base regularly fires high explosive munitions and Marines were doing just that, according to their website. However, she added, it was not likely that munitions training would be of the magnitude to cause a sonic boom.
An on-duty seismologist for the USGS confirmed to NBC 7 that there was no earthquake detected on sensors in San Diego County during the same time frame.
No further details were available Tuesday night.