A massive, abstract lookout tower providing a panoramic view of Prague.

One of the oldest, well-preserved water treatment facilities in all of Europe.

Bullet holes in the church wall are a reminder of the violence that followed the assassination of a high-ranking Nazi officer.

A giant interactive periodic table with real samples of the elements.

This spooky statue sits outside a famous Prague theater.

Looming communist pillar with a Czech twist.

The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

Street artists transformed an abandoned air vent into the beloved Star Wars robot.

Buried five stories underground, this Soviet bunker is packed with gas masks and Cold War paraphernalia.

This tipsy office building in downtown Prague was originally called “Fred and Ginger.”

A Jenga-style cylindrical tower of books that appears to go on forever.

The old observatory in Copenhagen has a spiral corridor used as a racetrack.

This sublime architectural marvel is a rare example of an expressionist church.

Secret Swing.

One of the largest underground systems runs under Canada's largest urban center.

This "Palace of Purification" has made many film appearances as a dark and sinister headquarters.

This massive, ancient stone was disassembled and then put back together again.

A memorial to the tens of thousands of Irish who fled to North America during the Great Famine.

Smallest house in Toronto sits on a lot meant to be just wide enough for cars to drive through.

Lost subway station nestled beneath Toronto's busy commuter line.

The remains of the concrete ship can be seen during low tide.

Two geological phenomena in one precarious locale: a towering stone window, and a deep blue void.

Hidden beneath a restaurant, underground tunnels tell the story of Malta’s tragic role in World War II.

The remains of a 1st-century aristocratic Roman town house featuring beautiful well-preserved mosaics.