Two people who lit a fire in a disused marl quarry cave in Zuid-Limburg could have died from carbon monoxide poisoning, local police said.

The two, who were discovered earlier this week, had built a fire in the so-called Douane cave near the city of Maastricht and were “completely scorched black”, police told local broadcaster 1Limburg.

“If they had stayed in the cave with the fire lit they would probably have died,” police said. In the event, they were checked for signs of carbon monoxide poisoning and taken to hospital. Police have not released a health update.

The pair were discovered by an official from nature protection organisation Natuurmonumenten who gave the alarm, resulting in what the police described as a “difficult scramble” to reach the cave.

There are some 250 corridor connecting the caves underneath the Limburg hills, the result of years of quarrying for limestone to make bricks.

“Most of the quarries are not safe and are closed off with barbed wire. The bigger corridor systems are some 80 to 90 kilometres long and have high gates and an alarm system. People who manage to get in risk getting lost,” said Natuurmonumenten spokesman Tim Coumans.

The caves themselves can be unsafe because they are not shored up, he said. They are also home to bat populations, and therefore off limits.

Despite the dangers, the cave are dry and warm and many homeless people choose to camp out there in the winter, Coumans said.

The caves also attract illegal raves, walkers, speleologists and people simply out to put a video on Youtube. Police fined some 10 people last year, Coumans said, but it is not known how many people enter the caves illegally. “If they are homeless it is not much fining them. We usually try to find a solution for them,” he said.