A marvel of Byzantine engineering under the streets of Istanbul.

A Beaux-Arts triumph set to become Ford's innovation campus

Cryptic messages embedded in the New York City sidewalk.

This Wall Street bar was once a hub of revolutionary activity where Washington famously bid farewell to his troops.

A very classy 1920s-era bar right in Grand Central Station.

Tucked away amidst some of the most famous addresses in the world is New York's only fraction of a street.

The remnants of two World's Fairs are here, complete with a 12-story globe, a mini-Manhattan, and a UFO-shaped pavilion.

The oldest surviving synagogue in New York City is now a soaring art space.

Oldest existing church in New York and the inspiration for Washington Irving's "The Legend of Sleepy Hollow."

A three-story copper skybridge connects two Manhattan buildings with Art Deco luxury.

Peek through the gates at this oasis from the city, full of Old English charm.

A semi-scholarly approach to sex.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

Elevated freight railway turned wildly successful urban park.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

A terrific purveyor of natural history objects and curios.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

It houses the remains of a 17th-century version of the Titanic.

A massive Montreal stadium that has seen nothing but trouble since its inception.

A grand Gothic Revival basilica with stained glass depictions of Montreal's religious history has only one soul resting in its crypt.

A recently restored Shoreditch landmark with ties to Jack the Ripper.

Hear the quietest sound from across the dome.

The remnants of a Gilded Age elegance preserved in the middle of a stark modern tower.

The world's largest piece of copyrighted artwork.