The curious underground history of keeping Paris clean.

The attic of a four-century-old canal house hides a clandestine church.

The art museum is decorated with creatively remodeled pre-Nazi swastikas.

Bored potato-guarding Nazis etched their thoughts onto a brick storage building in Copenhagen's Meatpacking District.

This beautiful train graveyard is one of the largest collections of steam locomotives in the Mediterranean and Europe.

This unusual elevator doesn’t stop or slow down.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

This gorgeous reading room houses the oldest and largest art history library in the Netherlands.

A floating cat sanctuary on an Amsterdam canal.

Cat art as far as the eye can see.

A curious collection dedicated to human and animal anatomy and pathology.

An Amsterdam street lined with antique shops each one dedicated to a different obscure collection.

This legendary dessert was served to Nobel laureates for decades.

Located in an old church, this brewery produces a different kind of holy water.

Every Tuesday, students let off steam by screaming into the night air.

Stockholm's narrowest alley is barely over two feet wide.

The single richest source of elemental discoveries in the world.

No one knows who's responsible for decorating this tunnel with stuffed animals, but the people love it enough to protest their removal.

Street artists transformed this abandoned industrial neighborhood into one of Europe's largest graffiti exhibitions.

Stockholm's deepest subway station is also a stunning ecological wonder.

The remains of Alfred Nobel's first nitroglycerine factory can still be found on the shores of a Swedish bay.

The internet as provided by a James Bond villain.

The library's graceful rotunda is like a bibliophile's temple.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.