The same mysterious tremors and low-frequency sounds that were observed on the Danish island of Bornholm and puzzled locals and researchers alike were also recorded in Blekinge County and the Baltic island of Oland in southern Sweden.
Seismologist Björn Lund, an associate professor at the University of Uppsala, cited reports of "low-frequency sounds and tremors" by locals, to the point of windows shaking. The past weekend, the phenomenon was said to last between ten and 13 seconds.
The Geological Survey of Denmark and Greenland (GEUS) ascribed the tremors to pressure waves from "an unknown source" in the atmosphere, adding more suspense to the story.
However, Lund pointed out that the same phenomenon has been measured several times a year in Sweden, including the Stockholm archipelago, and the tremors were found out to be caused by fighter jets breaking the sonic barrier.
"These are very much the same phenomena that we see here a few times a year. So for me it is the most proximate reason," Lund told Danish media.
According to Lund, the sonic boom may have appeared louder due the the phenomenon of inversion, where a different set of temperatures in layers of air above the Earth affects the spread of sound.
However, neither the Danish nor the Swedish military could confirm they had fighter jets on their wings on Saturday, the day the phenomena occurred last. Nor did the Swedish arms manufacturer Saab – the usual "culprit" in previous observations in Sweden, according to Lund – admit to performing test flights on Saturday. However, Polish authorities had previously disclosed intense military activity during the Anakonda23 drill in the northern part of the country, which involved a fighter jet and the live firing of artillery munitions.
Bornholm (population 40,000) is a rocky island in the Baltic Sea, south of Sweden, northeast of Germany, and north of Poland. It lies notably close to where the Nord Stream 1 and Nord Stream 2 gas pipelines were sabotaged in September 2022. Authorities in the relevant countries are still investigating the bombings.
A subsequent bombshell investigation by veteran journalist Seymour Hersh suggested the involvement of the US and Norway in the pipeline attacks; this has since been repeatedly rejected by the Biden administration. Western media outlets have instead hinted at a potential pro-Ukrainian group's role in the bombings.
Moscow considers the explosions at the two pipelines an act of international terrorism and will continue to use any available legal mechanisms to bring those responsible to justice , Russian Security Council Deputy Chairman Dmitry Medvedev said.