A roadway designed for a visiting monarch.

Looking strangely like Rome's Colosseum, this unfinished tower has dominated the skyline for over 100 years.

This historic hotel preserves many treasures from its heyday hosting passengers on the Orient Express.

The remains of the medieval city walls that once protected the Genoese quarter of Constantinople

A gorgeous, Gaudí-esque stairway constructed in the city of Istanbul by a prominent Jewish family.

Climb a steep hill in 90 seconds on the second-oldest urban subway in the world.

A marvel of Byzantine engineering under the streets of Istanbul.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

This incredibly diverse research station is one of the largest tropical botanical gardens in the world.

This fortress in Honduras was built to resist pirates, but its builders had more to fear from the British.

One of Mexico's youngest major cities still holds on to this structure representing its short modern history.

A massive underwater sinkhole made famous by Jacques Cousteau.

An oft-overlooked, magically musical monolith that stands majestically between Arlington Cemetery and the Iwo Jima Memorial.

This unique historical monument features large concrete spheres designed to resemble fallen stars.

At 207 feet, one of the world's longest continuous escalators.

This church built atop a gas station promises to "equip you for the next stage in your life’s journey."

A double agent hid information for the Soviet Union beneath this bridge for more than 20 years.

A derelict bit of transportation infrastructure hidden in the woods.

Art deco landmark on the outskirts of Washington, D.C.

The bank that helped fund the Mexican-American War and the purchase of Alaska met its downfall after helping Augusto Pinochet launder money.

America's Most Wanted's set resides in this tribute to the history of crime and punishment.

It was once the base for a series of domestic terror attacks.

A centuries-old stone pagoda from Japan stands among cherry trees beside the National Mall Tidal Basin.

An 18th-century townhouse that once hosted guests of the president.