A tsunami in Greenland left four people missing on Sunday, according to the Associated Press Monday. Nuugaatsiaq, Uummannaq and Illorsuit — three villages in the northwest region — were flooded, said the Greenland government.

According to Kalaallit Nunaata Radioa, the national broadcaster in Greenland, a possible cause of the tsunami is that ocean waves caused an earthquake on Saturday night. This may have resulted in Sunday’s tsunami. Video footage shows locals fleeing.

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Greenland has autonomy over domestic affairs but is part of Denmark. Denmark sent aid workers to help Greenland police. Although Nuugaatsiaq was evacuated, four people are still missing, Greenland police reported Sunday. Casualties were also reported.

The tsunami was not an isolated event. For years now, Greenland’s melting glaciers have caused earthquakes as icebergs break away and hit the glaciers from which they broke apart. It is common for these earthquakes to cause tsunamis. As melting continues, the tsunamis worsen, according to a study in The Cryosphere.

The Geological Survey of Denmark and Greenland (GEUS) reported that the earthquake that possibly caused Sunday’s tsunami had a magnitude of 4, as reported by the Danish news agency Ritzau.

“You have seen this in Greenland before,” Trine Dahl Jensen, GEUS senior researcher, said.

In July 2014, a notable tsunami was recorded by Martin P.  Lüthi and Andreas Vieli of the University of Zurich in Switzerland. The tsunami concluded at the glacier Eqip Sermia, a famous ocean-terminating glacier and a tourist attraction in southwest Greenland. The researchers utilized tide gauges and other instruments to analyze the ice collapse that caused the tsunami.

Video footage of the moment of the ice collapse shows the iceberg caving in, causing the water around it to ripple and form a wave.  In recent years, this glacier has lost exponential amounts of mass, following a trend of Greenland glaciers caving in and losing ice.

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“These glaciers are changing like crazy, really rapidly. Everything changes, and people cannot rely on their experiences from generations,” said Lüthi. “Suddenly things happen that nobody thought of before.”

A representative from Greenland’s police, Liselotte Boehm, spoke to Danish broadcaster TV2 about how her police force could not confirm the cause of Sunday’s tsunami.

“What we know is that some big waves have caused flooding,” she said.