A spectacular Bulgarian cave resembling a devil's head with a waterfall rushing down its throat.

This tiny Bulgarian town offers a stunning location in the mountains and a storied history.

This massive concrete sculpture is said to be the heaviest communist monument on Earth.

Bizarrely shaped rock formations in northwest Bulgaria come with equally bizarre legends.

An abandoned saucer-shaped monument in the Bulgarian mountains.

An architectural marvel that spans the Arda River, and has stories of a haunted past.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

This cubist Vienna church building looks like some sort of alien stonehenge.

Built to give thanks for the passing of a plague epidemic, this cathedral is a one-of-a-kind architectural hybrid.

A tropical paradise just a short trip from the opera house is filled with beautiful winged creatures.

The beautiful baroque library of the Hapsburg empire.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

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

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

A vast overgrown cemetery, once known as one of London's "Magnificent Seven."

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.

The verdant mossy ruins of a Franciscan monastery lie in the remote Sintra hills awaiting exploration.

This now abandoned restaurant provides some of the most amazing views of Lisboa.