An exclusive waiting room built for the royal family in the 1920s is hidden in Milan’s central station.

The odd depiction of the Virgin Mary and baby Jesus is rooted in the Catholic Church's eradication of Catharism.

An ossuary built in 1210 to collect bones from the Ospedale Maggiore of Milan.

The vast chamber inside this abandoned cooling tower looks like something out of a sci-fi dystopia.

Museum in a swimming pool where the water still flows.

This natural French spring has been turned into a magical grotto by Roman and medieval constructions.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

The Church of the Annunciation was carved into the face of a cliff, just like the rest of the town of Haute-Isle.

Ghosts of France's colonial past.

This crumbling, graffiti-covered mansion sits tantalizingly within reach of the proletariat who frequent a public park.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

This statue was given to France by the United States to commemorate the centennial of the French Revolution.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.

One of the oldest masonic museums in Paris.

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

A Belle Epoque bordello turned into a themed love hotel.

Pitié-Salpêtrière was the dumping grounds for women who received the dreaded diagnosis of "hysterical."

A haunting collection of the vanished and disappearing natural world.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

The splendor of this 17th century golden chapel has led it to be called the "eighth wonder of the world."

The loneliest island in New York.

The oldest surviving synagogue in New York City is now a soaring art space.