A marvel of Byzantine engineering under the streets of Istanbul.

This innovative steam machine was no doubt the most famous of the pioneer locomotives.

Graffiti from the 16th and 17th centuries cover the walls of this part of the Tower of London.

Tucked away in the Tower of London is the weapon once used to execute high-profile prisoners.

The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

19th-century scientists would make the pilgrimage here to verify the precision of their measuring sticks.

The first geometric self-supporting spiral stairs in Britain are a truly beautiful sight.

The handiwork of a underdog clockmaker who solved an impossible problem for an Empire, and forever revolutionized seafaring.

Thanks to modern navigational tools we now know that the true prime meridian runs through a park next door.

An amazing example of early-20th-century architecture, this church features marble transported from Utah.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.

Tiny memorials honor individual victims of the Holocaust.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

One of downtown Berlin's last reminders of its Communist past.

Beneath the impressive baroque cathedral that rises from the center of Berlin lies the largest collection of Prussian royals in the world.

The largest bell in the world has been swinging high above a Cologne cathedral for almost a century.

For decades, Puerta del Sol has been illuminated by a giant, guitar-wielding bottle of sherry.

An ancient Egyptian temple in the middle of Madrid, Spain.

The space-age building is home to one of the few remaining rotating restaurants in America, and the only one that inspired a 1980s sitcom.

A Trabant car, a kangaroo statue, and a plethora of plants take over this abandoned-factory-turned-bar.

A museum and shop on Bourbon Street located in the former home of the second Voodoo Queen of New Orleans.

Eastern Bloc nostalgia, anybody?