Series of underground houses inhabited continuously for centuries.

The largest underground church in Europe.

Historic medieval fortified city in southwest France.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

This French "sea of rocks" is packed with stunning rock formations and its own medieval castle.

A well-preserved medieval village nested on a rocky outcrop and surrounded by a network of cliffs, natural bridges and canyons.

This former quarry is now a picturesque gorge that hides a sunken lake.

This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

The deepest sinkhole in Central Europe comes with a grisly tale and an unusual nickname: “The Stepmother Abyss.”

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

Money made from smuggling sugar helped create this wondrous rock maze in honor of the Prussian Queen Louise.

An ancient monastery, a locked room, and a mysterious book heist.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

This natural French spring has been turned into a magical grotto by Roman and medieval constructions.

"The most colorful grottoes of the world.

A striking 12th-century-style castle built on the edge of a cliff.

Oberweissbacher Bergbahn - a broad-gauged cable car capable transfering railcars.

Abandoned railway line circling the city of Paris.

The Argonaute submarine was the French navy's pride and joy in the 1950's.

A strange rock formation with numerous legends trying to explain it.

On the highest peak in the Harz Mountains, science and the occult faced off in a failed attempt to turn a goat into a boy.

Goethe's "Faust" was inspired by these rock formations known as the "Devil's Pulpit" and "Witches' Altar."

The encased skull that is widely believed to belong to St. John the Baptist.

One of the few standing Roman lighthouses and a labyrinth of tunnels are a couple of the secrets at this historic castle.