PARIS (AP) — Two teenage boys have been rescued after three days underground in the skeleton-lined labyrinth of the Catacombs of Paris.

Paris police said they alerted firefighters early Wednesday that the boys, aged 16 and 17, were missing. Police said teams of rescuers, including climbers and sniffer dogs, set out to search for the youths and found them beneath southern Paris a few hours later suffering from slight hypothermia.

It was unclear how the boys got lost.

Sometimes people sneak into the catacombs after the official museum is closed and venture into areas normally off-limits, even organizing special parties or adventure games.

Some 20 meters (66 feet) below Paris, the catacombs hold the remains of 6 million people, transferred there starting in the 1700s as public graveyards were closed.