Hidden in plain sight on a pair of Manhattan subway tunnels are a pair of public instruments that sing to each other.

A peculiar mansion built by the troubled heir to the Winchester Rifle Company fortune.

An abandoned railroad tunnel is now filled with bioluminescent bugs.

Poet William Blake penned some of his most famous work during his brief stay at this beloved country cottage.

What looks like a normal building is really a hollow facade hiding a subway vent.

Original cabaret handbills, posters, and feline artwork are a purrfect highlight of the Museum of Montmartre.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

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

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

A museum of carnival attractions where you are invited to ride the rides.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

Notorious artist squat renovated into legal studios.

The forbidden island of the Tuscan Archipelago may still contain buried treasure.

The top of society once luxuriated in this world-class hotel, which continues to impress, even in ruin.

Crumbling ruins of the thermal baths that once attracted royals and aristocrats from throughout Europe.

Dug as a secret royal escape route, the tunnel became a wartime bomb shelter and dumping ground for vintage cars.

A domed disco abandoned since the '70s now attracts artists and explorers under its unique roof.

This Italian dam is covered in salt-hungry goats who manage to scale its almost vertical face.

This abandoned Brutalist treehouse is a hidden architectural relic.

Crushed coral and crystals give the sand its signature coloration.

This enchanting 600-year-old tree is so magical it inspired the beloved tale of Pinocchio.

This Italian foot path winds through the mountains via 52 individual tunnels.

The abandoned "Ghost Mansion" was left to decay in the mountains of Northern Italy.

The location of the last witch trials of Italy, known as "The Salem of Europe."