Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

Now harmless, this looming post struck fear into the heart of medieval troublemakers.

This picturesque medieval monastery guards a small trove of historic treasures.

Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

This scale model of our solar system hid in plain sight for years before a physics team sought out all the planets.

Where the lovelorn find a home for their sentimental keepsakes.

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 musical instrument played by the sea.

A whimsical bridge uses saxophones to pay tribute to two great men and the European Union.

This gruesome biblical parade celebrates the one day a year that a bit of Jesus' blood turns to liquid.

A beer pipeline runs beneath Bruges.

The bronzed skull of a beheaded traitor was affixed to the city gate in the 17th century.

An open-air sewer system that was covered piece-by-piece over 300 years.

Christ's blood allegedly preserved in a Belgian town.

A little sculpture with a large wardrobe.

From Tintin to the Smurfs, Europe's greatest graphic icons are celebrated in a grand Art Noveau gallery.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

Climb inside an iron crystal magnified 165 billion times its normal size.

The stunning interior of this Gothic church is covered in ornate gold details.

Elaborate tile work in this railway station tells the story of Portugal.

A trail of iron footwear stands as a monument to the thousands executed along this riverbank during WWII.

This bridge was constructed from the rubble of the infamous Bastille.

A series of abstract, silly sculptures represent the works of a great modern classical composer.