A vast reservoir of information on human health and behavior lies in our sewage, and this resource is untapped. We imagine a future in which sewage is mined for information that can inform policy makers, health practitioners, designers, and researchers alike. Such is the idea behind Underworlds: a cross-disciplinary data platform for monitoring urban health patterns, shaping more inclusive public health strategies, and pushing the boundaries of urban epidemiology. Pioneered by the MIT Senseable City Lab and the MIT Alm Lab, a prototype smart sewage platform is being developed consisting of physical infrastructure, biochemical measurement technologies, and the downstream computational tools and analytics necessary to interpret and act on our findings. The Underworlds project is the first of its kind, and a proof of concept that cities can make use of their wastewater system to do near real-time urban epidemiology and understand human health and behavior with a fine spatio-temporal resolution.