Two German children died after being buried under sand in northwestern Denmark on Sunday, Danish police said. The boys, aged between 9 and 12, were on holiday there with their parents.

They were digging a hole in the sand dunes on the beach. This probably caused the sand to shift. The dunes collapsed and the children were buried under the sand.

Police, local residents and firefighters then launched a search, and the Munich boys were pulled out of the sand after forty minutes of searching and digging. They were then taken to Aarhus Hospital in critical condition, where they died.

It is unusual for such a shift in the sand to occur, a professor from Aarhus University previously told Danish TV2. Sand avalanches usually occur mainly in February and March.

It is believed that the water was high, causing the waves to “bite” at the foot of the dunes. There was also heavy rain, which caused the sand to slide faster and bury the boys.

Police described the deaths of the two German teenagers as a “tragic accident.” The boys’ family said in a news release that they were grateful for the help provided by both local residents and emergency services.

On the same day, another sandstorm occurred in northern Denmark, injuring a 10-year-old boy. The boy was quickly rescued and was unharmed.