One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

Historic medieval fortified city in southwest France.

This massive blockhead sculpture hides an entire library.

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

San Francisco's first World's Fair, dedicated to the wonders of California.

The 80-foot tire that looms over Michigan's I-94 was once a Ferris wheel in the New York World Fair.

Climb inside an iron crystal magnified 165 billion times its normal size.

The crown jewel of the Queens Museum is a nearly 10,000-square-foot architectural model of the city originally built for the 1964 World's Fair.

What's left of the ruins of the 1893 Columbian Exhibition, or World's Fair, also known as the White City.

Designed by Buckminster Fuller, this relic of the 67 Expo survived fire and ice.

An eerie collection of the surviving structures from a past celebration of the future.

The world's largest wooden structure.

Single runway interrupted by a busy road that cuts across its middle.

Thousands of narrow alleyways in an ancient medieval city make up what's believed to be the world's largest car-free urban zone.

Traditional 11th-century outdoor tannery.

An awe-inspiring mosque in Morocco with one of the tallest minarets in the world.

The home of aviation trailblazers, in a remote corner of the big city.

Mysterious hillside with a continuously burning natural fire.

A shrine of fire worship built on top of a natural flame.

The building's fluid, wavelike walls are a true feat of architectural ingenuity.

This mysterious but iconic tapered tower appears in many Azeri fairytales and legends.

A massive collection of petroglyphs dating back some 40,000 years.

Home to nearly a third of the world's mud volcanoes, Azerbaijan is home to a messy, bubbling, and sometimes explosive landscape.

You'll find just about anything in this funky Tbilisi flea market.