While studies of social movements have mostly examined prevalent public discourses, undercurrents-the backstage practices consisting of meaning-making processes, narratives, and situated work-have received less attention. Through a qualitative interview study with sixteen participants, we examine the role of social media in supporting the undercurrents of the Umbrella Movement in Hong Kong. Interviews focused on an intense period of the movement exemplified by sit-in activities inspired by Occupy Wall Street in the United States. Whereas the use of Facebook for public discourse was similar to what has been reported in other studies, we found that an ecology of social media tools such as Facebook, WhatsApp, Telegram, and Google Docs mediated undercurrents that served to ground the public discourse of the movement. We discuss how the undercurrents sustained and developed public discourses in concrete ways.