The world's longest tunnel with an undersea section.

Single runway interrupted by a busy road that cuts across its middle.

Ancient paintings known as the "Sistine Chapel of Cave Art."

This impressive chasm is one of the longest natural underground cavities in the world.

Historic medieval fortified city in southwest France.

The largest bell in the world has been swinging high above a Cologne cathedral for almost a century.

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

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

A collection of art from Bulgaria's time as a communist state.

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

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

Sofia's legislative center is continuously reinventing itself.

A striking symbol of the Bulgarian capital.

A Bulgarian monument to Russia's role in World War II is constantly being painted in pop culture protest.

Climb to the top of its spiral spire for spectacular views of Copenhagen.

Danish tourist destination well known for open stalls where vendors sell marijuana on the street.

Hans Christian Andersen for the postmodern world.

The de facto symbol of Copenhagen is this bronze fairytale which cannot seem to keep its head for very long.

This contemplative sculpture made out of old machine parts is meant to be the first in a series of sculptures around the world.

Old Danish castle immortalized in Shakespeare's Hamlet.

Swedish skyscraper with a 90° twist.

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

An alien church provides much needed silence in the midst of a Finland shopping center.

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