A loud explosion-like sound was heard across many neighbourhoods of Helsinki late on Saturday night, but the source of the bang remained a mystery on Monday.

The boom prompted discussion on social media, from people in the neighbourhoods of Oulunkylä, Kumpula, Käpylä, Arabia, Kulosaari, Metsälä, Meilahti and Kulosaari claiming to have heard the noise at around 11:45pm on Saturday.

Speculation as to the source of the sound has also been broad. Some have suggested that it could have been a meteor crashing into the atmosphere, an earthquake or even a supersonic bang caused by an Air Force fighter jet. However, these possibilities have been all but ruled out.

Helsinki police began receiving reports from residents around the Oulunkylä district shortly before midnight. People described the noise as a “hollow explosive sound” and “a large gunshot.”

A police unit was sent to Oulunkylä but officers were unable to find a reason that would explain the sound.

Meteor, earthquake, war games ruled out

A group from the amateur astronomy association Ursa was also looking into the possibility that the noise came from a meteor incident. However, based on the images caught on the group’s network of cameras, there was no indication so far that the sound came from a meteor, according to Ursa spokesperson Anne Liljeström.

“The current opinion is that it was most likely not caused by a meteor, but of course that can’t be ruled out,” she said.

The loud boom was not related to Defence Forces operations either, the military agency’s communications chief Max Arhippainen told newspaper Helsingin Sanomat.

He told the paper that there was no military flight activity at the time the bang was heard that could have caused it.

Some have also pondered whether the explosive sound may have been related to the ongoing Zapad military exercises in Russia and Belarus. However, Arhippainen said that would be difficult to imagine since the drills were being conducted so far away from Helsinki.

Still others have speculated that the mysterious sound could have come from a seismic event like an earthquake. However, seismologist from the University of Helsinki, Tuija Luhta, said that most likely did not happen.

“In all probability, it was not any type of earthquake. We still have a more detailed analysis of that day [to examine], but we looked at it quickly over the weekend and it doesn’t show any clear [seismic activity] at our stations in Helsinki,” Luhta said.

“If it was an earthquake, it would primarily appear in large seismometers and would have been heard,” she said, adding that was not the case.