Mexico City's sewage system is a massive, complex engineering feat designed to prevent the sinking city from flooding. Most of the city's wastewater and runoff is channeled through a series of deep drainage tunnels, most notably the Túnel Emisor Oriente (TEO), which is over 62 kilometers long. This sewage is directed out of the Valley of Mexico toward the state of Hidalgo. Historically, this untreated wastewater was used to irrigate the Mezquital Valley, turning a semi-arid region into a productive agricultural hub, though this raised significant health and environmental concerns. To address this, the Atotonilco Wastewater Treatment Plant—one of the largest in the world—was constructed to treat approximately 60% of the city's discharge. In 2026, the focus has shifted toward "circular" water management, attempting to treat and reuse more water within the city to combat the severe water scarcity crisis and "Day Zero" threats facing the capital's nearly 22 million residents.