Almost 200,000 acres of land in the fertile Mezquital Valley are irrigated with the untreated sewage of Mexico City. Rainwater, urban runoff, industrial effluent, and sewage in Mexico City is sent to the Mezquital Valley through a 60-kilometer pipe. Soils in this valley have been continuously irrigated with urban wastewater since 1901, longer than any other soil in the world. The capacity of these soils to produce conditions in which agriculture can be safely practiced and produce healthy crops depends on complex interactions between soil composition, chemistry, ecosystem function, farming practices, public policy, land management, and the urban design of Mexico City’s hydraulic infrastructure. Without this wastewater, the Mezquital Valley would be a desert, as it falls into the UN’s “drylands” climate category, where rates of evapotranspiration exceed precipitation. Currently, more than 40% of the world is classified as “drylands,” making the Mezquital an important case study in wastewater reuse for a warming world. The goal of this research project is to evaluate the successes and failures of the Mezquital Valley as the world’s largest experiment fertilizing agriculture with human wastewater. What hydraulic, soil, ecosystem, social, and policy conditions increase chances for success? What are its prospects for socio-ecological sustainability? In order to address these questions, we will partner with farmers, urban water experts, and soil scientists in order to understand both sides of the wastewater pipe as part of the same agricultural system. This research will produce an advanced studio curriculum at Harvard University on the architecture of postnatural soils.