This chapter uses the 1930 Meuse Valley incident, when 63 people were killed in Belgium due to toxic industrial gases mixing with a dense fog, to explore historical and methodological questions about rumour, fear, science, and war during times of crisis. Exploring the intersection between modernity, scientific understanding, and human behaviour, the chapter demonstrates how rumours created different systems of knowledge and scientific inquiry. The chapter highlights how rumour construction in the Meuse Valley illuminates the significant psychological impact of chemical warfare on populations, the continuity of pre-existing belief systems in the face of scientific analysis, and the persistence of international mistrust. The chapter also raises questions about the parameters for trusting knowledge, and how parameters can change in times of crisis or war. These questions are particularly pertinent in an age of post-truth politics, the attempts of national governments to control social media, and the regulation of information and ‘fake news’.