A series of caves under Castle Hill that once held the prisoner Vlad Tepes, better known as Count Dracula.

The remains of an ancient Roman city are hidden underneath Bologna's beautiful public library.

Over 100 towers were built in Bologna during the Middle Ages, but only 22 survive.

This modernist tower was saved by a fraternity with a 500-year-old penchant for shooting at wooden parrots.

A World War II evacuation tunnel turned into a history display of Croatian state and people.

This military tunnel dates back to World War II and provides visitors with a little wartime adrenaline.

A musical instrument played by the sea.

A rooftop restaurant hides in plain sight atop Vienna's Palace of Justice.

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

A silly medieval mural preserved on the side of a Viennese house.

Legends surround an ancient tree trunk riddled with nails for good luck.

Baroque celebration of the end of the Great Plague of Vienna.

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

The topiary maze at the Habsburg hunting lodge.

A tropical paradise just a short trip from the opera house is filled with beautiful winged creatures.

The final resting place for the remains of 143 Habsburg royalty.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

The beautiful baroque library of the Hapsburg empire.

This historic electric train has operated with regularity since the early 1900s.

The 2,000-year-old Roman columns are hidden within Barcelona's Gothic Quarter.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

Gaze upon chocolate sculptures depicting everything from chariot races to Gaudí's famous church at this shrine to sweets.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.

A beautiful but toxic fountain of mercury.