Update, Jan 10: Marblehead and Salem have also experienced "mystery explosions." One resident in the area captured video of one on a security camera. It seems a lot like a firework. Ball Square residents: Is this similar to the explosions you saw?
Across the country, a loud explosion heard in North Hollywood is also a mystery, according to the LA Times blog. As in Somerville, police and fire officials were not able to identify the cause.
Residents of Evansville, Ind., and other towns in that area, were "jolted out of bed late Monday night" by "what sounded like a loud explosion," according to WTVW Channel 7. As in Somerville, officials thought it could have been a blown transformer, but it wasn't.
Across the Atlantic, the community of Gresford, in the United Kingdom, also experienced strange early-morning explosions in late December.
Residents who live near Ball Square were woken up at around 4 a.m. Wednesday by loud explosion-like noises.
One resident who contacted Somerville Patch described them as a "terrifically loud series of four bangs" and said the concussions set off a car alarm. They sounded nothing like gunshots, the resident said, but rather like "really big flash-bang fireworks rockets."
They weren't the only ones to hear the noises. Deputy Chief Charles Breen of the Somerville Fire Department said he personally heard the bangs.
Thomas Champion, a spokesperson for the city, said in an email that Somerville's 311 system received four calls about the noises, but "there were a number of other calls reporting the noise" that went to 911 and the police department's non-emergency line.
According to Champion, people reported the noises as coming from the area around Willow Avenue, Foskett Street, Kidder Avenue, Lowden Avenue and Bay State Avenue, all in a neighborhood close to the Brown School. There was one outlier: Someone reported hearing the noises around Forster Street, almost a mile away in Winter Hill.
In addition to the noises, "Some callers apparently also said they saw an accompanying bright white flash," Champion said.
The fire department dispatched Engine 6 to the area, thinking there might have been a transformer explosion, but "we came up with nothing on our end," said Breen.
The Somerville Police Department sent four units to investigate, and the city also called NSTAR, according to Champion.
"No cause was identified," he wrote.
For those of you who believe in ghosts, you might find an explanation at the nearby Powder House, which is apparently haunted by an angry mill owner whose arm was crushed in the old mill. according to Myths and Legends of Our Own Land, written in 1896 by Charles Skinner.
If anyone has information about the bangs or a theory about what they were, let us know.