In the Pacific Islands, whose people contribute the least to global warming, but are facing its most severe effects, impacts from climate change are far from a future scenario. Here, in the great maritime region of Oceania, the future is today. Sea-level rise and erosion of land combine to create a challenging situation not documented historically: entire nations risk the partial or total loss of their land, and the fate of their ocean territories as Exclusive Economic Zones defined by extent from the land is uncertain, as is the citizenship of those whose low-lying islands are at threat. In this situation the countries of the Pacific represent themselves as Big Ocean States as a powerful alternative to the UN category of SIDS (Small Island Developing States), and engage in global diplomacy and activism to safeguard their ocean. The MARE NULLIUS project aims to build new and urgent interdisciplinary analysis of one of the greatest challenges of our time: how Pacific islanders and their states prepare for, and react to, what may be the demise of their sovereign land and sea territories.