A turn-of-the-century architectural marvel built on healing mineral springs and salt licks.

This medieval castle overlooking the Elzbach river looks just like it did hundreds of years ago.

The mountaintop inspiration for Umberto Eco's "The Name of the Rose."

Underwater restaurant at the coral reefs of Eilat.

Pagan monument converted to Christianity with the population.

Floating swimming pool within a moored vessel.

Stunning Roman aqueduct used for 1900 years and still standing.

Mediterranean castle on a tiny Turkish island.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

A series of modernist treehouse hotel rooms grace the outskirts of an Arctic forest.

This 14th century castle withstood years of battle but is now sliding off the hills into the Danube.

Remnants of a storybook amusement park hidden behind a strip mall.

Invisible bridge parts the waters, invoking biblical images and preserving the ominous feel of an ancient moat.

No need to strap in for a "ride" on this walkable roller coaster.

A massive sustainable farm designed by the mastermind behind Central Park.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

World War II-era Nazi compound nestled in the Santa Monica hills.

French Time Capsule-19th Century home revealed to the public.

A stunning, abandoned limestone quarry in southern Indiana provided the stone to erect the Empire State Building.

An abandoned village that doubled as "The Hunger Games"' post-apocalyptic wasteland is up for grabs.

Abandoned Coast Artillery fort now serves as a graffiti canvas and sightseeing vantage point.

A dilapidated dock tells the story of Titanic's missed arrival.

Scattered throughout London are ancient remnants of the city's former bounding wall.