Two stone columns flank the grand Venice square, but there were supposed to be three.

Go inside to really appreciate this 500-year-old technological masterpiece.

The only remaining arch of the oldest stone bridge in Rome stands in the middle of the Tiber.

The remains of this 2nd-century Roman temple are incorporated into a modern building.

This curious, round temple is the oldest still-standing marble building in Rome.

This is the oldest Roman-made bridge in the city still existing in its original state.

An ancient Roman temple first dedicated to an emperor's wife is now a church.

This ancient market is often referred to as the oldest shopping mall in the world.

This large, circular tomb is the final resting place of the first emperor of Rome.

An abandoned Gold Coast-era mansion once intended for the exiled ruler of Albania, who never arrived.

A forgotten relic from New York's bygone trolley system hides in plain sight under a bridge.

Stunningly detailed mosaics in a New York subway station bring life to the Weimaraners of artist William Wegman's work—and to thousands of commutes.

A sculptural commentary on capitalism pokes up from the East River.

A 9-story, poured concrete structure which was used as a glue factory with an indelible smell.

A historic lighthouse steeped in rumors of ghost and pirate stories.

Hollywood's most famous landmark, the "temporary" sign that has stood for decades longer than intended.

Why have you never heard of the second most popular tourist site in New York?

An indoor miniature village and labor of love for Laurence Geiringer, a model railroad enthusiast.

Belly up to the bar in Cowtown, don't forget to buy some boots.

The only surviving prototype for Buckminster Fuller's house of the future.

The highest point in the state of New Hampshire, and while unlikely "the worst weather in the world," it sure seems that way to hikers.

The highest point in Washington state is a volcano covered in glaciers.

One part local history, 99 parts gruesome haunted house.

The king of U.S. high points towers over every other peak on the continent.