Hundreds of hidden passageways weave through the old quarters of Lyon.

A cave dwelling that was started by the Celts, expanded by monks, and finished by knights.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

The first green space constructed on an elevated viaduct.

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.

This museum houses an imposing skeleton army comprised of creatures great and small.

France's national museum of scientific and industrial instruments.

A favorite Paris bar for U.S. expats, including Gershwin, Hemingway, and James Bond.

Walk upon the mementos of the dead in this Parisian courtyard paved with medieval tombstones.

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

The very center of Paris is surrounded by a bevy of strange good luck rituals.

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

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

Abandoned railway line circling the city of Paris.

The oldest scientifically dated tree in Europe lives on a remote cliff.

A medieval Italian village is fighting depopulation with this vibrant annual street art festival.

This tiny islet has somehow housed an ancient temple, two monasteries, a church, a fortress, and a restaurant.

Now an entertainment complex, the Lingotto building was once an avant-garde auto factory with a test track on the roof.

This seaside Italian village doesn't just look Escher-esque, it actually inspired the mind-bending artist.

Ride a boat through this magnificent underground world, which now doubles as a theater.

One of the most well-preserved castles in Italy also contains a rare medieval example of a port fortification.

The Emperor Tiberius' palace of debauchery.

Thousand-year-old Viking runes adorn an ancient Greek lion at the entrance to the Venetian Arsenal.