Down a cobblestone alleyway is a museum dedicated to Romanticism, with a secret garden.

BDSM, 1930s Style.

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

France's national museum of scientific and industrial instruments.

19th century pendulum and a clock restored by a rogue group of guerilla artists.

A Belle Epoque bordello turned into a themed love hotel.

The headless bodies of more than 1,300 guillotined victims of the Reign of Terror are buried here in mass graves.

Notorious artist squat renovated into legal studios.

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

Got apples?

Paris' fabled purveyor of exotic taxidermy and natural history curios.

The curious underground history of keeping Paris clean.

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

Known as the "Dodo Manège," this carousel features a herd of animals that have disappeared—or may be gone soon.

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

The personal library of the founder of modern neurology is open to visitors.

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

This amazing abuela became a diehard metal fan late in life and an icon of Madrid's '80s rock scene.

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

Old car parts and assorted materials get turned into one man's mechanical treasures on the side streets of Cleveland.

Some of the earliest leaders of the Moravian congregation are buried here.

According to legend, this tomb holds the secrets to time travel.

Over 12,500 words are scrawled across each room of this historic plantation house and no one knows why.

Though illegal, city officials have made an exception for this work of art.