Ancient paintings known as the "Sistine Chapel of Cave Art."

A complex, nine-layer maze on the grounds of a Venetian doge's playground.

The mental institution where Van Gogh created some of his most famous work now allows guests to walk in the artist's footsteps.

Macabre astrological automaton clock dating to the late 1400s.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

The "Prussian Versailles" that is as glorious today as when it was built.

The world's most active stratovolcano is almost always erupting.

Italian cave with exceptional acoustics and an unsavory history.

Beneath the city of Lourdes, France is a massive, concrete church.

Eighty million natural history specimens call this gargantuan museum home.

A beautiful but toxic fountain of mercury.

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

Furniture tries to escape an abandoned building in San Francisco.

San Francisco's newly reopened hands-on science museum.

Staircase winds down the side of Telegraph Hill through gardens, wild parrots flying overhead.

The former home of the Nobel Prize-winning American writer is now home to dozens of six-toed cats.

Two stunning 17th-century libraries and a cabinet of curiosities.

Oldest building in California still in use.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

Yale University's home for rare works, including the mysterious Voynich Manuscript.

In Egypt's expanding metro area, many of the neighbors are dead.

These medieval skyscrapers dominate the skyline of this small Tuscan town.

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

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