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

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

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

Thousand-year-old Viking runes adorn an ancient Greek lion at the entrance to the Venetian Arsenal.

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

Originally a leper colony, the island is now an Armenian monastery that's home to an incredible manuscript library and eclectic museum.

A sinister pyramid in the Frari contains the heart of the famous neoclassical sculptor.

This Venice bookstore has resigned itself to constant flooding by keeping its books in bathtubs and boats.

An extraordinary house museum and cabinet of curiosities in the heart of Florence.

A slightly creepy gallery of plaster cast models in the same museum as Michelangelo's David.

A medieval weapon rumored to have been used by the brilliant poet himself.

Over 50 exquisite hand-painted Renaissance maps decorate a Medici storage room.

A large grotto built in the 16th century amidst Florence's famous Boboli gardens.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

A 15th-century monumental Latin inscription on this house celebrates Lorenzo Manilio's love for Rome.

Legend has it that this bronze orb, which sat atop an ancient Egyptian obelisk, once held the ashes of Julius Caesar.

Hidden within this iconic mountain are a set of World War II bunkers once used by German forces.

Roman politician Herodes Atticus built this nymphaeum to honor his wife, whom he may have murdered.

A forgotten "city of water" underneath the famous Trevi district.

Roman-era ships unearthed...

This large, circular tomb is the final resting place of the first emperor of Rome.

A modern sliver of architecture in Roma.