Alternate Reality Games (ARGs) are multiplayer role-playing games that use the real world as their primary platform and incorporate a range of media, including video, audio, email, mobile technologies, websites, live performance, and social networks. This paper describes the development, implementation, and player reception of "The Source," a novel ARG designed to spark STEM interest and learning among youth from populations traditionally underrepresented in STEM fields. "The Source"'s design was informed by the existing literature on recommendations for engaging underrepresented populations in STEM careers, specifically emphasizing social support systems, technology, transferable skill-building, and the relevance of STEM topics to youth from underrepresented groups. Our preliminary evaluation sheds light on some of the highlights, concerns, and lessons learned from implementing this youth-oriented ARG for out-of-school STEM learning.