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

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

Curious cave that stays a cool 56 degrees all year long.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

Giant granite rocks standing end-to-end like a train of circus elephants.

Erosion and time have created a naturally-formed water park.

European castle ruins in an American state park are actually the product of death and grief.