The source of mysterious sounds that caused anxiety for residents of Greece’s northern port city of Thessaloniki has been discovered, according to media reports.

A team from the city’s water supply authority (EYATH) discovered the sound was the result of an anomaly in water flow through pipes, said the Iefimerida news outlet.

Residents have been jolted awake since last December by the banging noise they describe as thunderous and even unearthly.

Agis Papadopoulos, the president of EYATH, told the Greek news outlet that sound was coming from the routine tuning of the water supply running underneath the neighborhood of Meteora, where the sound was most prominent.

“Imagine that the water flows through the pipes like a wave. If there is some sort of anomaly in the flow, the contact between the water and the pipe can produce a sound,” he said.

Papadopoulos urged residents to remain calm and said technical teams are carrying out adjustment works.

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