Much of human experience can be distilled to saltwater: tears, sweat, and an enduring connection to the sea. In Vast Expanses, Helen M. Rozwadowski weaves a cultural, environmental, and geopolitical history of that relationship, a journey of tides and titanic forces reaching around the globe and across geological and evolutionary time. Our ancient connections with the sea have developed and multiplied through industrialization and globalization, a trajectory that runs counter to Western depictions of the ocean as a place remote from and immune to human influence. Rozwadowski argues that knowledge about the oceans—created through work and play, scientific investigation, and also through human ambitions for profiting from the sea—has played a central role in defining our relationship with this vast, trackless, and opaque place. It has helped us to exploit marine resources, control ocean space, extend imperial or national power, and attempt to refashion the sea into a more tractable arena for human activity. But while deepening knowledge of the ocean has animated and strengthened connections between people and the world’s seas, to understand this history we must address questions of how, by whom, and why knowledge of the ocean was created and used—and how we create and use this knowledge today. Only then can we can forge a healthier relationship with our future sea.