Vulcan's Project Halo is an ambitious, multiyear research program to develop a detailed scientific knowledge base that can answer AP-level questions and provide explanations in a user-appropriate manner. It is one of the largest AI research programs in the US today. Halo's current focus is building AI tools that allow graduate students in chemistry, biology, and physics to author scientific knowledge adequate to answer sophisticated natural-language questions without relying on trained knowledge engineers. Halo researchers have been working to link Semantic Web technology with the other knowledge representations in the system. This talk will lay out Halo's technologies and results to date, and describe the technical and UI issues we have faced in getting users to author scientific conceptual knowledge.