This one-time health resort was used by aristocrats around the world.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

The Argonaute submarine was the French navy's pride and joy in the 1950's.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

A subterranean shelter remains intact under a busy Paris railway station.

How did a French journalist’s grave become the sexiest statue in the cemetery?

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

This Parisian café sticks almost exclusively to an ethos of reuse.

The curious underground history of keeping Paris clean.

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

The history of radiation is on safe display at the foot of a radiation science laboratory.

Indents in the pavement are all that remain from a grisly death machine in Paris.

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

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

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

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

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

France's most famous cemetery, with some of its most curious tombs.

Notorious artist squat renovated into legal studios.

The birthplace of most all of Europe's space missions is here on the Dutch coast.

A lecture hall sink once used by Einstein and other noted physicists is now regarded as a strange science relic.

Now a museum, this building was once a 14th-century monastery and the place where the founder of the Dutch nation lived and was assassinated.

The world's first interactive human body museum also serves as a chair for a giant orange man.

Devious political maneuvering and four ancient, working telescopes are central to the world's second-oldest observatory.