STEM learning is an integrated approach to improving learners' problem-solving capacity and 21st-century skills by engaging them in systematic investigation that requires interdisciplinary knowledge. This study aimed to examine whether the design of an innovative knowledge-building environment facilitates STEM learning. Participants were university students engaging in in-depth group projects to design a piece of living-technology product. Data were obtained from student groups' online discussion of their STEM projects. Both quantitative and qualitative analyses of student groups' knowledge-building activities, including fostering a strong sense of community, working productively with ideas, and assuming higher-level agency, provided evidence of students' deep engagement in the design of their STEM projects. Recommendations for the design of effective STEM learning environments are offered.