The world's longest known cave system.

A castle built by an eccentric medieval enthusiast in rural Ohio.

A turn-of-the-century architectural marvel built on healing mineral springs and salt licks.

Once one of the world's great cities, Cahokia was a place of religious worship, trade and mass human sacrifice before being mysteriously abandoned.

A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

Small, privately owned island divided between Michigan and Ohio with abandoned vacation homes and a derelict lighthouse.

A medical facility that has been kept frozen in time.

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

Home with Gothic windows featured in a famous painting.

This freshwater spring fills with 10,000 gallons of water per minute.

Full-sized barn houses many restored instruments.

Large pioneer village showcasing life during boom times.

An oddly poignant roadside attraction celebrating American folklore.

These pretty kitties killed and ate as many as 135 railway workers.

The subtle science of a master molecular gastronomist.

Shop of curiosities with a dinosaur museum tucked in its basement.

A bizarre house filled with an astounding array of collections.

This striking art museum holds an amazing series of curiosity cabinets.

A place to eat for on-the-run agents and espionage enthusiasts.

"World's largest scrap metal sculpture" stands in an outdoor sculpture garden in the middle of Wisconsin.

A large circus museum with summer performances.

Eternal resting place of the grave robber and murderer who served as inspiration for many of cinema’s famous madmen.

Complex aquaponics systems allow fish and plants to grow in harmony.

Rising 30 feet in the air atop Quarry Hill, the Old Powder House is the oldest stone building in the state of Massachusetts.