Roman ruins top white travertine terraces formed by ancient hot springs.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

Every summer, an open-air swimming pool appears at the base of a Finnish city's beloved winter attraction.

If you visit Santa's village, you can get your passport stamped to show you crossed into the Arctic Circle.

Roughly 17 million letters to Santa have found their way to this Arctic Circle post office.

A 246-foot-high revolving restaurant rises unexpectedly from a forested Finnish hilltop.

An abstract monument to a symphonic composer.

A subterranean house of worship carved right out of the bedrock.

Sofia's legislative center is continuously reinventing itself.

A vast Communist-era exhibition space in the Bulgarian capital.

"Holy Wisdom" overlooks the city that became its namesake.

A gigantic statue of a Russian soldier watches over the ancient city of Plovdiv, a prominent reminder of the Soviet period in Bulgaria.

A magnificent, glittering building tucked into a tiny Bulgarian mountain town.

"Unity, Creativity, Beauty."

Eastern Bloc nostalgia, anybody?

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

A striking symbol of the Bulgarian capital.

This historic black and white monastery is one of Bulgaria's architectural jewels.

The ceiling of Prohodna Cave in northern Bulgaria affords a peek at the sky via two deity-sized-eye-shaped holes.

An abandoned saucer-shaped monument in the Bulgarian mountains.

Bizarrely-painted church designed by a controversial artist.

The housing complex served as an anti-fascist fortress during the Austrian Civil War.

The world's largest historic armory was established by independent citizens to defend themselves from the wars of larger forces.

Austria's ultra-modern art museum looks like Cthulhu's giant alien heart.