Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

The last of Amsterdam's original sea level markers remaining in place.

Some 80,000 Delft Blue tiles line the inside of this Amsterdam walkway.

A set of seven houses, each built in a style that represents a European country.

A former bank seized as a sniper vantage point during the Bosnian War is now a secret graffiti art gallery.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

This mural painted by Keith Haring was done without payment as a gift to entertain the children at the Hôpital Necker.

This bridge was constructed from the rubble of the infamous Bastille.

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

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

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

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

The celebrated eyesore created by starry-eyed sweethearts.

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

The oldest continuously operating mint in the world that has been making coins since the 9th-century.

The approximate spot where the last Grand Master of the Knights Templar was put to death.

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

A 16th-century fountain marks the site of a lost medieval cemetery once filled with mass graves.

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

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

Notorious artist squat renovated into legal studios.

Paris' linguistic guide to the international language of, yes, love.

This scale model of our solar system hid in plain sight for years before a physics team sought out all the planets.

A war-torn Olympic relic is being overtaken by graffiti and nature.