How did the Japanese prepare against disasters in the past? What kinds of knowledge influenced countermeasures? How did disaster prevention (bōsai) become a government responsibility? This book explores the origins of Japan’s disaster prevention regime by focusing on scientific experts from various disciplines, including civil engineering, seismology, geology, meteorology, and urban planning. These experts played a crucial role in shaping disaster prevention measures, legislation, and education, participating in the global scientific and technological developments of their time on the one hand while managing societal expectations on the other. The sense of political urgency created by disasters served as an argument for altering and shaping the environment in the name of disaster prevention. For the experts, generating knowledge in an environment shaped by disasters also created a scientific niche through which Japan could gain an international reputation as a nation that successfully deals with disasters. This book sheds light on how extreme natural conditions, bureaucracy, science, and technology have shaped modern Japanese history and provides insights into how environments have structured the global circulation of knowledge.