A museum dedicated to the history of electric interurban transit near the home of the first electric trolley system in the United States.

Peek into a cave of giant, glimmering crystals.

Since it formed millions of years ago, this cave system has been a sacred burial ground, a Prohibition-era speakeasy, and the center of an amusement park.

An underground wonderland in Iowa.

Once one of the most polluted caves in America, now an environmental success story and a museum dedicated to caves.

Ride a boat through this magnificent underground world, which now doubles as a theater.

The spectacular geological formations make it one of the most impressive caves in Europe.

Before it was buried by volcanic ash, this fast-food restaurant fed the hungry hordes of ancient Rome.

Isak Heartstone, a 15-foot wooden troll made out of recycled materials.

A former waterfront market is now in the center of town due to some interesting Boston engineering.

An Ancestral Puebloan ruin that appears to burst into flames in the sunlight.

Enormous lake at the bottom of a unique cave system.

The largest and most well-preserved petroglyph site in the Verde Valley.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

Ancient serpentine sacrifice holder that has stood the test of time.

A coal fire has been burning underground here since 1896.

Elegant marble and old vaults fill this historic Dutch bank turned office space.

Aged nearly 1,700 years, the world's oldest bottle of wine is probably still drinkable.

A sharp sculpture made of thousands of pieces of jagged glass.

Where dogs like the rare "Fire-Tailed Border Cocker" run free on the mountainside.

The remarkably well-preserved final resting place of a powerful ancient Thracian king.

The ancient necropolis houses the immense rock-cut tombs of four Persian kings.

Did Inca children also slide down these smooth rocky slopes above Cusco?