This article analyzes “not knowing” and overlooked or invisibilized information related to the drinking water system in Lima, Peru, a rapidly expanding megacity of the Global South in a region considered to be at high risk to climate related hazards. Within this system, unavailable, or disregarded information exists for a range of topics, from quantitative information about reservoir levels to more qualitative concerns about decision-making and administration. Currently, information made publicly available focuses on end users, specifically: census data on percent of homes connected to the municipal network and statistics comparing consumption patterns across neighborhoods. This deflects attention from topics related to water supply (expansion of urban hydraulic reach, infrastructure investment), and decision-making and control. However, this article considers a type of “not knowing” underlying all of the above, which is the forgotten or overlooked development of Lima's drinking water system over time. It presents results of a study of the history of this system from its origins in the 16th century through modernization in the early- and mid-20th century, concluding with infrastructure investment in the era of climate change. It focuses on continuities in the spatial orientation of water infrastructure as well as long-term conflicts over its administration, to demonstrate how the current system is the result of a centuries-long process of accretion, rather than a system designed to serve the current urban population. Finally, it concludes with a discussion of the significance of this infrastructural and conceptual inertia for understandings of Lima's water risk and vulnerability.