This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

This 22-floor tower's Skydeck offers panoramic views of Amsterdam.

This historic Amsterdam bar used to accept sailor's monkeys as payment.

Peek through a transparent pyramid and drink in the mountain made of books.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

This small town in the Netherlands has no roads but instead, miles of canals and over 100 bridges.

An unusual cave filled with rare alien-looking aragonite formations.

Just a few miles from Novalja, a strange geological oddity has left scientists baffled.

This beautiful Croatian fortress is haunted by legends of forbidden love.

Colorful concentric circles honor a small Croatian town's supposed spot at the nucleus of it all.

Now harmless, this looming post struck fear into the heart of medieval troublemakers.

This Croatian island is for sale but no one is buying due to the dozens of accused Nazi sympathizers who were massacred there after WWII.

This abandoned Croatian stronghold never saw a great deal of conflict but it is still a beloved relic nonetheless.

This scale model of our solar system hid in plain sight for years before a physics team sought out all the planets.

This former estate of a fascist leader is now a burnt out ruin leading to a system of secret tunnels.

Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

The only gulag ever to exist in socialist Yugoslavia.

A full figure 4th century incorruptible saint lies in a glass coffin in Croatia.

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

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

Legend has it fairies gathered at the site of this medieval castle.

This stunning, out-of-the-way Croatian monument commemorates a community's uprising against occupying fascist forces.

A three-shelf library serves as a memorial in the Jewish cemetery of Krems.

A series of haunting wooden totems are hidden in the woods surrounding a German environmental center.