Ivy-covered Medieval tunnels cut through this charming town in France's Loire Valley.

The castle was reconstructed to fit Kaiser Wilhelm II's romanticized vision of the Middle Ages.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

This massive monument towers over the city as a constant reminder of the lives lost during Algeria's hard-won war for independence.

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

Seven mini rowhouses fill the gap between 54 and 70 Westerstraat.

Part of an abandoned19th-century fortress hides in plain sight inside a modern office building.

A fantastical 13th-century fortification sandwiched by a modern hotel in central Brussels.

This fantastic cave remained unexplored until 1948 when three schoolboys discovered a hidden entrance.

Stockholm's deepest subway station is also a stunning ecological wonder.

This hidden collection of everyday objects illustrates life within communist Poland.

The Netherlands' first car tunnel has an unusually rectangular shape.

A fearsome dragon clings to a corner of Munich's city hall.

The first high-tech company in the valley that actually worked with silicon devices.

Where Mary Shelley’s "Frankenstein" and John William Polidori's "The Vampyre" were born.

A rare, flower-shaped example of Brutalist architecture in Switzerland.

This irregularly shaped sea fortress has defended not one but three different sovereign states.

This statue of a Hindu cosmic dancer at CERN is a metaphor for modern physics.

The World Wide Web was created in this nondescript corridor at CERN.

Home to the world’s oldest seismograph and a four-ton steel ball that makes its own earthquakes.

Elegant marble and old vaults fill this historic Dutch bank turned office space.

Four-story slides send people whizzing through the math department at this Munich university.

A rare tour of an aristocrat's private home reveals valuable 17th-century paintings from artists like Rembrandt.

This whimsical statue dumps water on its head to symbolize knowledge flowing into his brain. Or maybe booze.