The last chapter, “Cabled Depths: The Aquatic Afterlives of Signal Traffic,” analyzes how undersea cables exert a lasting influence on our knowledge about and inhabitation of the ocean. The chapter documents the relationship between early marine science and telegraph cable networks, which together helped chart a distinct set of transoceanic paths. During the era following World War II, these exchanges were increasingly shaped by the U.S. militarization of the seafloor, and cables took on a new function as they were mobilized for the acoustic monitoring of marine space. Today these systems feed back into the development of marine scientific research via the construction of cable-linked ocean observatories, and into new extractive relationships with the seafloor. Although the first five chapters focus primarily on the manipulation of physical sites and social practices, chapter 6 explores the institutional and epistemological interconnections that have inflected cable development. In describing how our knowledge of the ocean is thoroughly intertwined with cable histories, the chapter—like the ones before it—reveals the porous boundary between communications technologies and their environments.