Dutch police on Tuesday found five siblings and a man believed to be their father, who had been locked in a house on a farm for as long as nine years.
The five, estimated to be between 18 and 25 years old, and a man they identified as their father were found near Ruinerwold, a village in the northern province of Drenthe.
"We found six people living in a small space in the house which could be locked," police said. "It is unclear if they resided there voluntarily."
Police said the people could have been locked away in the property for nine years.
Media said the family was found after a man, 25, who was dirty and unkempt entered a nearby cafe in a confused state and urgently pleaded for help.
Officials later found a secret staircase behind a cupboard that led down to a basement in the farmhouse, media reports said.
Local news station RTV Drenthe said the six young adults "have been living in a basement for years, waiting for the 'end of times'".
On Tuesday, local Mayor Roger de Groot said a man, 58, who was not the father of the children, had been arrested. His role was unclear.
The freed people were suffering delusions, with some of them having no idea that other people existed.
"I have never come across anything like this before," Mr de Groot said.
“Police investigated after receiving a tip-off from somebody who was concerned about the people's living conditions and discovered the adults. They lived an isolated lifestyle."
The children's mother had apparently died before they moved to the Dutch farm, the mayor said.
None of the family members were registered as residents with the municipality, police said.
Drenthe police are investigating the circumstances.