An Empire of Air and WaterPlanetary spaces such as the poles, the oceans, the atmosphere, and subterranean regions captured the British imperial imagination. Intangible, inhospitable, or inaccessible, these blank spaceswhat Siobhan Carroll calls "atopias"existed beyond the boundaries of known and inhabited places. The eighteenth century conceived of these geographic outliers as the natural limits of imperial expansion, but scientific and naval advances in the nineteenth