An 80-year-old plane tree is devouring an iron bench at Ireland's oldest law school.

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

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

Sitting on an Italian hilltop is this surreal fortress built by the mad inventor of electrohomeopathy.

2300-year-old Roman road connecting the empire.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

A landmark in the history of technology and computing is hidden in a tiny Baroque chapel.

When this funny little building went up in the 19th century, most people thought it would simply fall over.

A fake medieval village in the middle of Turin.

This shining multicolored metro station deep under Naples is said to be the most beautiful in Europe.

A serene space tucked amid the skyscrapers of Hong Kong's bustling financial district.

The world's longest outdoor covered escalator system transports commuters through Hong Kong's hilly terrain.

In Hong Kong, a fairytale flower tunnel is hidden within the gardens of a public housing complex.

Remnants of the most densely packed city in world history, that thrived on anarchy for 30 years.

This hulking behemoth is actually five interconnected towers built in the 1960s.

A giant wooden soda bottle pays homage to a distinctly British drink on the very spot where it was invented.

These former Nazi anti-aircraft towers offer a singular view of the north of Berlin in the summer, and a sanctuary for bats in winter.

Cold war icon and onetime "hole" in the iron curtain.

A restaurant and observation deck built in a unique broadcasting antenna.

A museum dedicated to the history of a terrifying secret police is now held in their former headquarters.

A slice of central Asia in the middle of Berlin.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

The Pope’s secret escape route.