A musical instrument played by the sea.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

An ethereal glowing blue ocean cave in Croatia.

A spectacular Bulgarian cave resembling a devil's head with a waterfall rushing down its throat.

The ceiling of Prohodna Cave in northern Bulgaria affords a peek at the sky via two deity-sized-eye-shaped holes.

This tiny Bulgarian town offers a stunning location in the mountains and a storied history.

This historic black and white monastery is one of Bulgaria's architectural jewels.

Bizarrely shaped rock formations in northwest Bulgaria come with equally bizarre legends.

An architectural marvel that spans the Arda River, and has stories of a haunted past.

There's a mini version of Iguazu Falls hidden in the Balkans, and it's got a rope swing.

This museum preserves the hand-dug tunnel that kept Sarajevo afloat during the Serbian siege.

Bridge that died in war and was resurected.

This cluster of little wooden huts once ground local farmers’ wheat into flour during the days of the Austro-Hungarian Empire.

A 14th-century fortified keep showcases the masterpieces of an ingenious 20th-century clockmaker.

This abandoned Belgian fortress is purposefully being left to nature.

Theme park featuring miniature replicas of European monuments.

One of the deepest indoor swimming pools in the world.

A spherical "micro-nation" in the heart of Vienna.

"World machine" built by an untrained Austrian farmer.

The Habsburgs' royal collections of art and wonders.

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

The world's largest collection of intentionally useless inventions.

Located right outside of Vienna, visitors can take in expansive views of an ethereal forest.

Ride a dragon train into a world of dwarves.