In global oceans, circulating current systems called gyres concentrate floating plastic waste into garbage patches far from land. This dissertation describes how the Great Pacific Garbage Patch accumulating between California and Japan comes to matter as an environmental problem and public concern at the turn of the 21st century. It draws on participant observation, interviews, historical and textual analysis to "follow" plastic as it circulates - with water, images, people, knowledge and marine life - from the ocean, through laboratories and beyond. By tracing the intersecting trajectories of multiple materials, I take a problem often blamed on activist exaggeration or media misrepresentation and show how the garbage patch emerges with a diversity of collective practices. The production and sharing of knowledge not only shapes the garbage patch, but also the kinds of solutions and care that are possible in return. For some, the garbage patch becomes a solid 'trash island' twice the size of Texas in need of cleanup; for others, a whole new realm of inseparable associations between synthetics and life called the plastisphere. Plastic, however, continues to escape from these attempts to measure, know, cleanup and otherwise control it, challenging the cultural and political foundations of science and ecology. I argue that caring for the ocean requires responding to plastic in all its natural-cultural relationships, as it transforms humans and environments alike
