The oldest stone house in Paris was built by its most famous alchemist.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

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

The only residents of this abandoned housing development full of identical castles are stray cats and dogs.

The best-preserved ancient theater in the world.

While not Turkey's deepest underground city, this subterranean redoubt is the largest.

This Turkish cave boasts some rather bizarre associations with the King of Mount Olympus.

Flames have been burning within this mountain for more than 2,500 years.

A marvel of Byzantine engineering under the streets of Istanbul.

This unreal Portuguese palace looks as though it's made from a pile of different castles.

Rare wooden escalators lead deep down into an otherwise staggeringly monotonous tiled tunnel.

This stunning gothic ruin was a casualty of the Indiana steel industry crash.

A mysterious concrete hand reaches out from between the stones of Uniwersytecki bridge.

The spectacular history of spectacles on display.

The mirrored section takes the shape of Hebrew characters when viewed from above.

Where royalty waits in the height of luxury.

This 1920s Amsterdam neighborhood was built as a giant experiment in concrete construction.

A set of seven houses, each built in a style that represents a European country.

In the days before house numbers, these colorful signs held clues about various buildings' purposes.

A rare tour of an aristocrat's private home reveals valuable 17th-century paintings from artists like Rembrandt.

Amsterdam's former hotspot for public executions and dissections.

A poignant museum dedicated to the history of the Dutch resistance against Nazi occupation

Tucked in the back of a flower shop, its collection details the fascinating history of the Netherlands's iconic symbol.

A unique collection of gilded age mechanical pianos.