(Bloomberg) -- Inhabitants on Denmark’s Baltic Sea island of Bornholm, the site of last year’s Nord Stream explosions, were last weekend spooked by tremors that the Nordic country’s authorities said they can’t explain. 

Seismologists first thought the May 13 event stemmed from an earthquake or a controlled explosion in nearby Poland, but have now ruled out those possibilities after further analysis of the data, according to a statement on Monday by GEUS, the Geological Survey of Denmark and Greenland.

The tremors were “most likely acoustic pressure waves from one or more events somewhere in the atmosphere,” GEUS said in the statement. “However, it’s not possible for the researchers to conclude what caused these powerful pressure waves.”

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