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

This picturesque medieval monastery guards a small trove of historic treasures.

A musical instrument played by the sea.

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

Trimmed trees grow alongside the graves like topiary tombstones.

The Renaissance-era courthouse is a relic from the fortified town's centuries of Venetian rule.

Croatia's "hairy stone" fortress is one of the most well preserved Istrian castles.

Once a state-of-the-art tuberculosis treatment spa, these ruins are now ruled by paintball players.

Where the lovelorn find a home for their sentimental keepsakes.

A forgotten monument to Yugoslavian freedom.

Now abandoned, the world's first torpedo factory is crumbling into the sea.

Humble beginnings for one of the world's most famous scientists.

This huge metal monument is slowly being stripped to its concrete skeleton.

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

The haunting concrete remains of a Nazi pebble crusher can still be found in this small French commune.

Folklore and history carved into Brittany's coast.

A passage grave used in neolithic rituals.

This natural crater along the coast of Jersey may take its name from the devil statues who have watched over it.

A statue erected in honor of the toadish nickname given to Jersey residents by the French.

A wooden statue of an Estonian pagan god and King of the Setos stands atop a sacred hill.

This old Estonian mine has been kept like a sprawling time capsule of the local shale industry.

Legend says lovers who hold hands while venturing along this stone jetty and kiss at the end will remain together forever.

A close look reveals that the Scottish monument is just slightly off-kilter.

Now famous as the "Harry Potter bridge," this impressive concrete viaduct has long been an icon in its own right.