This Luxembourg cave is covered in cryptic looking marks left by mining activity.

A historic cave in Scotland once served as a hiding place for a famous Covenanter.

The "Black Arch" guards the entrance to the picturesque Glens of Antrim.

Modern megaliths form this work of cosmic, Celtic landscape art.

The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

Hiding the longest subterranean river known in the world.

Incredible labyrinth of man made caves.

The home of Ireland's last leprechauns is officially protected by the European Union.

Miles of man-made caves and tunnels are currently used for cultivating mushrooms.

A series of idealized caves carved to entertain guests.

An 80-year-old plane tree is devouring an iron bench at Ireland's oldest law school.

Limestone grottoes turns everyday objects into stone.

Figures of unknown origin carved into the surfaces of underground caves.

Each spring, a dreamlike carpet of bluebells overtakes the forest floor.

The largest cave chamber in Britain—big enough to fit a whole cathedral inside.

Harry Potter once visited this massive limestone cliff, which was carved by an ancient glacial river.

Abandoned railway line circling the city of Paris.

Once thought to be the mysterious work of a witchy prophetess, a well with unusual properties turns objects into "stone".

The remains of centuries of copper mining appear like an alien landscape.

A subterranean playground in an abandoned slate mine.

These crude contraptions were used to aid in the mining of lead, silver and gold.

Semi-precious Blue John stones have been mined in this cavern system for centuries.

A magnificent winding road through a steep valley, with deep natural caverns at the bottom

Some of England's last cave dwellers lived and worked within this huge cave.