Archives and oral histories show that the Ming Tombs Reservoir was a showcase project in Communist China directed by and involving the country's top leaders. This was one of the first projects to rely on the mobilization of physical labor rather than specialized machinery, driven by a belief in self-reliance and the use of local resources. It argues that the focus on the "masses," rather than engineers or scientists, challenged established engineering procedures and technical traditions. Historical evidence suggests that adopting a "build while being designed" mindset and mobilizing the "masses," projects could be completed, but often in ways that ultimately proved less than optimal. The case study suggests that innovations fail when local enthusiasm and technical knowledge are not balanced. By focusing on the role of the "masses" in shaping a novel technological landscape, this article highlights "mass engineering" to better understand this model of native innovations and economic autarky.