This book furthers academic scholarship in cutting-edge areas of geographical and geopolitical writing by drawing on a series of little-studied undersea living projects conducted by the US Navy during the Cold War (Project Genesis, Sealab I, II and III). Supported by an engaging and novel empirical setting, the central themes of the book revolve around the practice and construct of 'territory', 'terrain', the 'elemental' and the interrelationships between these material phenomenon and both human and non-human bodies. Furthermore, the book will point to future research trajectories in the form of 'extreme geographies' to better understand living practices in a world that is increasingly submerged and extreme.