For a brief 56 years, around 1205 to 1261 A.D., Paquimé, Mexico (also called las Ruinas Casas Grandes) was in its prime, a city about 280 km northwest of Chihuahua in a basin on the vast high plains of northern Mexico. Paquimé relied upon many innovative hydrological systems—perhaps including a geothermal district heating system begun around 1060 A.D. If this proves true, the Paquimian system is the oldest geothermal district heating system in the world. How the city came to be as it was—and why its golden era was so brief—are integral parts of a complex hydrological story that includes geology and cultural history