A perfectly circular lake is formed by the crater of an ancient underwater volcano.

This abandoned five-star hotel is now a five-star urban exploration destination.

The medieval castle built as headquarters for the Pope's secretive Knights Templar.

An ancient and majestic fortress that played an integral role in Portugal's road to independence.

A small Portuguese cottage formed between boulders in the countryside.

This fairy-tale, neo-Gothic church in the Azores is a world of magical realism.

A unique seaside cave where Aleister Crowley faked his own death.

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

From Iggy Pop to David Bowie to Nick Cave, this regal music studio has served some of the all time greats.

The music is over at the Ballhaus Riviera Ballroom, but the ballroom remains in haunting disrepair.

The most visited, and possibly most controversial, museum in Germany.

An Art Nouveau gem hides beneath the layers of city grime obscuring this once-magnificent building.

This ancient city - home to nine civilizations - is soon to be buried under 200 feet of water.

Over the centuries, this island paradise has done it all, from providing monks with ecclesiastical respite to sheltering an exiled Leon Trotsky.

An ancient church on a tiny island covered in stone bible stories has controversially been turned into a museum.

Fantastic lake on the heart of a fault line.

A meow-velous tribute to a beloved street cat, made famous on the internet.

A sacred lake with thousands of sacred carp and a biblical history.

Standing proud atop a Turkish bluff, this Ottoman palace has retained its grandeur despite being abandoned for centuries.

A Turkish monastery tucked into a cliff wall.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

Once described as the "Eagle's Nest," this ruined ancient city rests on a remote mountaintop.

Ottoman-era houses lovingly preserved.

Naturally protected city carved from a rock face.