This article examines how the Korean government’s decision concerning the import of US beef sparked massive candlelight protests for several months in 2008 and beyond. Emphasizing indeterminacy of participation, this article characterizes the unpredictable and emergent formation of public groups online and offline as networked publics and their diverse and unpredictable ways of participation as multitentacled. Because scientific claims regarding mad cow disease (BSE) were among the central issues galvanizing participation, I pay close attention to how networked publics constructed collaborative expertise to counter government logic. Conceptually, this article analyzes recent discussions of public participation in STS and then highlights the indeterminacy of its participation and the mutual process of constructing scientific claims shared by protesters and experts. Empirically, this study primarily depends on a variety of data elicited from observation, interviews, and various literatures. In the discussion section, I raise questions concerning how we should understand public participation differently in STS and how the Internet age brings us new forms of politics and participation.