An underground city of salt.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

This bridge was constructed from the rubble of the infamous Bastille.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

A phallic shrine to a princess goddess in Krabi, Thailand.

One of the Seven Wonders of the Modern World holds the world's tallest metal staircase.

This extraordinary geological oddity resembles human-made pavement.

Come for the flowers, stay for the bats!

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

A sealed test tube said to hold Thomas Edison's dying breath was given to the inventor's friend and mentee, Henry Ford.

Controversial tribute to the heavyweight champion boxer who grew up in Detroit.

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

A building so large that it has its own weather.

An icon among Florida's pre-Disney tourist attractions, just west of Orlando.

A tribute to Ponce de Leon's supposed quest for eternal youth.

A 56-foot tall statue of the god of fire looks out over Birmingham, Alabama.

A large circus museum with summer performances.

A pop-culture attraction outside of Dyersville, Iowa, hits a home run.

Most comprehensive museum in America devoted to one ethnic group.

Giant six-pack could provide one person a six-pack a day for 3,351 years.

Half-sized replica of the Leaning Tower of Pisa built to honor Galileo.

The world's longest known cave system.

Moonbows over Kentucky.

Enormous lake at the bottom of a unique cave system.