Addressing crises sometimes requires grappling with sophisticated technical or scientific content. To make sense of the BP DeepWater Horizon Oil Spill people had to grapple with uncertain and sometimes contentious, complex information. This empirical study shows that an emergent, connected crowd interacted to surface, share, question and discuss these complexities. While studies have observed collective sensemaking taking place via social media in other kinds of crises, this study extends our understanding of emergent crowd work as collective sensemaking where members of the public assemble and interpret evidence on complex topics in a crisis context, perhaps performing a kind emergent citizen science.