No need to strap in for a "ride" on this walkable roller coaster.

A sprawling Baroque park with 300-year-old hydro-pneumatic waterworks.

These 12th-century catacombs carved into a mountainside were a place of mysticism and Christian hermitages.

An Austrian house of bones filled with hundreds of intricately painted skulls.

The world's oldest known salt mine.

A breathtaking viewing platform over the Alps.

In between two buildings in Ljubljana, medieval tunnel to nowhere.

A miniature train-ride through Slovenia's largest cave system.

This massive subterranean fissure is known as the "underground Grand Canyon."

These pools have been abandoned for over 20 years.

The walls of this medieval church are covered with perfectly preserved frescos, including a pristine example of the Dance of Death.

Croatia's "hairy stone" fortress is one of the most well preserved Istrian castles.

One of the best-preserved Roman amphitheaters is still used today, but for less brutal events.

Now abandoned, the world's first torpedo factory is crumbling into the sea.

A Bulgarian monument to Russia's role in World War II is constantly being painted in pop culture protest.

An abandoned saucer-shaped monument in the Bulgarian mountains.