The largest underground church in Europe.

This massive blockhead sculpture hides an entire library.

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

Ancient paintings known as the "Sistine Chapel of Cave Art."

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

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

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

Science fiction's grandfather has a fittingly marvelous tomb.

A surrealist timepiece by the artist of melting clocks.

A haunting collection of the vanished and disappearing natural world.

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

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

Former hunting lodge of a duke who was an avid collector of impressionist works.

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

Two awe-inspiring globes from the reign of the Sun King hang from the ceiling of France's national library.

This life-size replica of the Statue of Liberty's flame is also a tribute to Princess Diana by default.

This innocuous-seeming building is actually the remnant of a major wartime brothel.

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

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

A Belle Epoque bordello turned into a themed love hotel.

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

The curious underground history of keeping Paris clean.

A majestic lion statue guards this Parisian botanical garden, and feeds on an unsettling item.

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