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

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.

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.

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

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

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

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

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

Abandoned railway line circling the city of Paris.

One of Antoni Gaudí’s earliest buildings, built between 1886 and 1890 for the industrialist Eusebi Güell.

Saint Joseph was said to heal the deaf and blind, but it was a much smaller miracle that got him canonized.

Barça football fans might see the insignia hidden in a church window as a divine sign.

Barcelona's only remaining tram system takes riders to the Tibidabo funicular.

A vista of the city and sea from atop the little-known viewing deck on an unassuming Columbus statue.

The iconic boulevard is the perfect place to catch sight of a bicycling skeleton, a levitating Satan, or a chef with his head stuck in a pizza.

This cafe offers a curated collection of treats and tipples made by monks and nuns.

A beautiful but toxic fountain of mercury.

This now abandoned restaurant provides some of the most amazing views of Lisboa.

This charming candle shop has been in business since 1789.

This unique architectural wonder is also tied to one of the 20th-century's most renowned writers.

One of the smallest bookstores on the planet.

This sculpture of a bowler hat is complemented by a matching umbrella a few blocks away.

This downtown Dallas art installation is always on the lookout.