Yugoslavia's biggest underground airport lies in ruins.

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

A World War II evacuation tunnel turned into a history display of Croatian state and people.

A well-worshipped shrine to the Virgin Mary nestled inside the last of Zagreb’s historic gates.

This 135-year-old weather station studied the effects of the climate on human health.

Hidden within this iconic mountain are a set of World War II bunkers once used by German forces.

A fascinating assortment of cave dwellings, medieval ruins, and sarcophagi are being devoured by the forest.

This ancient altar emerges from the thick forests to reveal the mysteries of Etruscan sacrificial rituals.

A 16th-century horror show built in a lovely Italian garden.

This archaeological site built into a tuff outcrop is filled with rock dwellings, the ruins of a medieval castle, and a necropolis.

Water and windows define this defensive Italian well.

This picturesque Umbrian city has a subterranean side that has long remained hidden.

A formerly glacier-tipped volcano that inspired none other than Jules Verne.

This black church sits alone among a field of lava rock.

Instead of solid circles, the stoplights in this northern Icelandic town have glowing red hearts.

In Northeastern Iceland lies something of an oxymoron: a steaming desert.

A sulfuric martian landscape that looks far better than it smells.

This elegant arc of a waterfall marks a religious turning point.

The most powerful waterfall in all of Europe.

This canyon is comprised of gorgeous waters and magnificent basalt columns.

A giant red chair bolted to a rock on a remote mossy hillside in Iceland.

This replica Viking village was created for a film that was never shot and is now open to visitors.

A black sand beach where glacial fragments wash ashore and look like sparkling diamonds.

Iceland's largest lagoon is home to stunning multi-colored icebergs.