This massive library holds what was once the largest book collection in the world.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

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

This heart-shaped island looks substantially less lovely after a recent bulldozing project.

An ancient knight and a symbol of the city's freedom.

A self-proclaimed libertarian micro-state on a tiny bit of disputed territory between Serbia and Croatia.

Croatia's towering concrete blossom grows from the site of a World War II concentration camp.

A wunderkammer of gold and silver reliquaries and ancient saintly bones.

This Croatian fairytale castle is slowly falling apart thanks to years of neglect.

Longest fortress system anywhere in Europe.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

A former bank seized as a sniper vantage point during the Bosnian War is now a secret graffiti art gallery.

Celebrated example of Ottoman architecture that also served as a site for brutal massacres.

The remains of the last Bosnian fortress to fall to the Ottomans are still standing.

A bridge destroyed three times: once as a ruse, then in an actual attack, and finally for the movies.

This historic cemetery was inaccessible for several years due to dozens of land mines.

The haunting ruins of a military fortress turned astronomical observatory sit on a Sarajevo mountaintop.

Bridge that died in war and was resurected.

A war-torn Olympic relic is being overtaken by graffiti and nature.

These warring psychedelic crags commemorate an unsuccessful military operation.

This exclusive resort has completely covered a tiny Balkan island in a 5-star warren of guest rooms.

These historic defenses ensured the city within would survive hundreds of years of occupations, sieges, and invasions.

This ominous Albanian fortress houses tanks and downed planes as a reminder of their victories over Western imperialism.

A medieval bridge stretches over a sulfur-rich river that leads to year-round natural hot springs.