How has the concept of a world ocean emerged in a world of difference? This question reveals the crucial problematic of planetary environmental politics, which attempt to contend with global-scale environmental crises caused by the human species, at the risk of ignoring geographical specificity and different ways of knowing and experiencing life on Earth. By tracing the emergence of the world ocean concept in international oceanographic science, my research explores key practices and paradigms that characterize this tension. Drawing on expert interviews as well as archival materials from the US, UK, and South Africa, I study the world-making practices of three projects in physical oceanography: the International Geophysical Year (1957-58), the World Ocean Circulation Experiment (1990-2002), and the Global Ocean Observing System (current). Finally, I suggest the ocean archive as an alternate role for the ocean in planetary thought.