The 1940s structure houses what was once Europe's largest telescope.

Looking strangely like Rome's Colosseum, this unfinished tower has dominated the skyline for over 100 years.

This palatial building houses a remarkable gallery of papal portraits.

The sanctuary seems to either float atop the water or sink into the dirt, depending on the water level.

In 2013, historians discovered a hidden room devoted to the art of making royals' hot chocolate.

A galley-shaped memorial to the prisoners of an infamous fortress.

A pair of stones embedded in an Edinburgh street memorialize one of the city's last public hangings.

This church carved into the rocks is full of enchanting stone statues.

A lavish 16th-century mansion filled with spectacular staircases, fantastic frescoes, and gorgeous gardens.

Beer lovers thirsty for knowledge can now visit a museum devoted to the world's largest hop-growing region.

The approximate spot where the last Grand Master of the Knights Templar was put to death.

This structure atop a craggy cliff in one of the most beautiful parks in Paris boasts an unbeatable view.

A beautiful Victorian statue shows France's enduring love of the rhinoceros.

The video game-themed street art first popped up in the French city.

This emotive sculpture was originally created for the 1878 World's Fair.

Geological storytelling.

Indents in the pavement are all that remain from a grisly death machine in Paris.

The only substantial part of Nazi Germany's formidable defensive line through Northern Italy that survives today, largely intact.

Fortified medieval tower house with lovely roof garden.

A map of Italy made entirely of trees.

A line of abandoned, alien-like fortresses dot the mountains of North Italy.

The only visible reminders of an elaborate engineering project that kept the tower from reaching the tipping point.

London's smallest police station is barely the size of a phone booth.

A historic house left in "dramatic still life."