8. Casa Grande Ruins (Arizona)
Casa Grande Ruins, located near Coolidge, Arizona, is a four-story adobe structure built around 1350 AD by the ancient Hohokam people. The “Great House” remains the largest known prehistoric earthen building in the American Southwest. Constructed using caliche, a natural concrete-like material, the structure stands 60 feet tall and 60 feet wide. Its purpose remains a mystery, though it may have served as a residence, ceremonial center, or astronomical observatory. The site was designated a National Monument in 1918 and is managed by the National Park Service. (nps.gov).
