The 2013 Kumbh Mela, a Hindu religious festival and the largest human gathering on earth, drew an estimated 120 million pilgrims to bathe at the holy confluence of the Ganga (Ganges) and Yamuna rivers. To accommodate the massive numbers, the Indian government constructed a temporary city on the flood plains of the two rivers and provided it with roads, electricity, water and sanitation facilities, police stations, and a tiered healthcare system. This phenomenal operation and its impacts have gone largely undocumented. To address this gap, the authors undertook an evaluation and systematic monitoring initiative to study preparedness and response to public health emergencies at the event. This paper describes the water, sanitation, and hygiene components, with particular emphasis on preventive and mitigation strategies; the capacity for surveillance and response to diarrheal disease outbreaks; and the implications of lessons learned for other mass gatherings.