This book is about the extreme sport of marathon swimming. It provides insight into a social world about which very little is known, while simultaneously exploring the ways in which the social world of marathon swimming intersects and overlaps with other social worlds and configurations of power and identity. Drawing on extensive (auto)ethnographic data, Immersion explores the embodied and social processes of becoming a marathon swimmer and investigates how social belonging is produced and policed. Using marathon swimming as a lens, this foundation then provides the basis for an exploration of what constitutes the 'good' body in contemporary neoliberal society across a range of sites including charitable swimming, fatness, gender and health. The book argues that the self-representations of marathon swimming are at odds with its lived realities, and that this reflects the entrenched and limited discursive resources available for thinking about the sporting body in the wider social and cultural context. It argues that in spite of these constraints, novel modes of embodiment and pleasure seep out between the cracks of those entrenched understandings and representations, highlighting the inability of the dominant discourses of sporting embodiment to account for the experience of immersion. This in turn opens up spaces for resistance and alternative accounts of embodiment and identity both within and outside of marathon swimming. The book is aimed primarily at readers at undergraduate level and upwards with an interest in sociology, the sociology of the body, the sociology of sport, gender and the sociology of health and illness.