Ancient limestone quarry tunnels are decorated with generations of graffiti. During WWII the tunnels were converted for use as an air raid shelter, but were never used. Fossils of a prehistoric Mosasaurus and turtle discovered here are now on display at the Maastricht Natural History Museum.
One tour per day in English.
October 7, 2011
Muraglione, Italy
Ride a boat through this magnificent underground world, which now doubles as a theater.
Peñíscola, Spain
The blowhole bursts spectacularly right next to the buildings in a medieval seaside town.
Quebrada, Puerto Rico
An enchanting natural wonder in Puerto Rico's Camuy River Park.
Mialet, France
This army of concretions is an unexplained geological phenomenon unlike anything else in the world.
Da Nang, Vietnam
This Vietnamese hell cave lets visitors take a walk through a devilish underworld full of demons.
West Liberty, Ohio
These sub-farmland caves are home to a majestic stalactite known as the "Crystal King."
Kato Kastania, Greece
This remarkable cave was discovered after a shepherd watched bees enter and exit through a small fissure.
Kansas City, Missouri
The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.
