A modern-day Lascaux cave underneath Paris.

Abandoned railway line circling the city of Paris.

Love is in the air along the small street in Brooklyn.

The San Marino-Rimini railroad was short-lived but this tunnel once provided shelter for hundreds of refugees during World War II.

The largest living museum in the Northeast recreates life from the Early Republic period complete with costumed historians and reenactors.

The unique design of these skyscrapers in a Paris suburb led to them being nicknamed "cloud towers."

A suburban housing complex with astonishing architectural features, locally nicknamed "the Camemberts."

After traveling the world, the last remaining fully-rigged three masted iron-hulled cargo ship in the world found its home in New York.

This tower is one of the best-preserved (and least-visited) remnants of medieval Paris.

One of London's lost rivers runs through a pipe over the tracks of this subway station.

An interim housing alternative to FEMA trailers that is designed for densely populated areas during recovery and rebuilding following a natural disaster.

This statue of one of the engineers who created the London Underground hides a ventilation shaft for the tube lines that run under the street.

The French leader's hat sits in a glass case at the oldest restaurant in Paris.

The bones of the Biblical Magi traveled to Constantinople and Milan before landing in their current home inside an elaborate golden reliquary in the Cologne Cathedral.

The site where two doves fell in love.

The location of this small park was once the site of an infamous public morgue.

Abandoned power station once home to a squatter community, now being developed into an arts center.

This stunningly massive housing complex is a go-to filming location for dystopian sci-fi cinema.

This renovated Art Nouveau bathroom from 1905 is worth the price of admission.

Marie Antoinette and Napoleon Bonaparte were fans of this 200-year-old French chocolatier.

Montmartre's sole surviving windmill became an iconic landmark for 19th-century painters, artists, and bohemians.

The only female equestrian statue in Washington, D.C. is a 15th-century French heroine.

The cave of a 12th-century hermit is still visited and venerated today.

This translucent sculpture represents a quintessential component of New York City's skyline.