The way that light hits a stained glass window on a specific day is one of many "clues" to a mysterious treasure.

One of Jeff Koons’s largest sculptures and his first commemorative work.

The woman who helped save 15th-century France was born in this humble home.

View one man's eccentric collection of nearly 5,000 corkscrews, then sip some Chablis.

Open since 1977, this small museum is filled with a lifetime of finds from a tireless traveler and collector.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

Notorious artist squat renovated into legal studios.

Abandoned railway line circling the city of Paris.

Gigantic Romanian salt mines used to heal respiratory conditions.

Romania's triumphal arch looks out over Bucharest.

No one knows who built this strange cave of worship or exactly why.

This snug medieval lane is one of the narrowest streets in the world.

Something's watching you.

This once-derelict building is now one of the most beautiful bookstores in the world.

This enormous abandoned prison was once dubbed the "Bastille of Romania."

This 11th-century structure is one of the oldest churches in Romania.

After sitting unused for decades, this abandoned communist infrastructure became a wildlife utopia.

Visit an imposing 13th-century Transylvanian citadel lurking not far from the region's more heavily touristed attractions.

This fortress-like monastery is covered with intricate frescoes.

An abandoned pasta factory in the centre of Bucharest, once home to Bulgarian revolutionaries and a hub of crime.

A chemical factory damaged by two earthquakes and an explosion that became abandoned after bankruptcy.

An old spa town in the mountains of Romania where, according to legend, the great hero Hercules once bathed.

Locals believe this abandoned ruin on the outskirts of Bucharest is haunted.

This elaborately decorated, fairytale-like Romanian royal palace is one of the most striking castles in Europe.