The world's longest known cave system.

Walk the whole Mississippi River!

Labor provided by Great Depression-era programs restored these 25,000 acres in Tennessee.

Mile-long cave system near Great Smoky Mountains.

The mysterious wonder of an underground waterfall is illuminated by multicolored lights.

Visit this playground of waterfalls, caves and sinkholes.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

Home of an evil witch famous within southern folklore.

Enormous lake at the bottom of a unique cave system.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

The remains of an idyllic small town built for the film 'Big Fish' still stand around a rundown main street.

This natural wonder drops 16 stories straight down through rare flora and streaming waterfalls.

400 million-year old limestone caverns nestled in the Ozarks served as shelter and a hideout for cultures throughout North American history.

When you can afford only half a road, you build only half a road.

This former mining town was named the most toxic town in America.

In the middle of a suburban Denver neighborhood is a cave that once provided solace for a prominent Ute chief and his family.

This super steep hike ascends 2,000 feet in less than a mile.

A little-known hot spring haven right below the highway.

Community owned and operated, this property is home to a diverse set of nudists.

Abandoned silver mine strewn with wrecked and twisted structures.

The highest bridge in America was built for an almost unnervingly small amount of money.

Hidden beneath a bridge, this waterfall is known for the large amounts of graffiti painted on its base.

This naturally-sourced hot spring is now home to a clothing-optional resort.

A breathtaking array of ancient Pueblo structures and artifacts counterintuitively preserved by neglect.