Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

This top-secret nuclear bunker hidden in a Bosnian mountain has been converted to a modern art exhibition space.

Minnesota's highest point is a scenic vista marked by a plaque and armies of mosquitoes.

Half of a waterfall goes into the river while the other half seems to disappear.

Duluth mineral magnates built this charming art deco lodge to be their private clubhouse.

The only gulag ever to exist in socialist Yugoslavia.

Underground monastery and fortress.

Ancient rock-hewn town that witnessed key events of Georgian history.

A collection of items from a way of life that no longer exists.

Despite being pillaged and destroyed a number over times down the centuries, this historic church still sort of stands.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

A toxic ghost town sitting atop a massive coal fire.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

An eerie, derelict amusement park in Berlin.

A fiery crater has been burning in the Karakum Desert since 1971.

An artsy hidden staircase leads to breathtaking views of San Francisco.

A church of bones, decorated with 40,000 human skeletons.

An exquisite tunnel of cascading flowers.

Rippling sand dunes frozen in the Arizona rock.

A magical place that lets you play in and explore the world of Studio Ghibli.

An emerald-green grotto just a short trip from Austin.

La Monte Young and Marian Zazeela's "Dream House" will immerse you in an ever changing world of sound and light.