Through stories of history, culture, and animals, the author explains how water temperature, salinity, gravity, and the movement of Earth's tectonic plates all interact in a complex dance, supporting life at the smallest scale--plankton--and the largest--giant sea turtles, whales, humankind. From the ancient Polynesians who navigated the Pacific by reading the waves, to permanent residents of the deep such as the Greenland shark that can live for hundreds of years, she introduces the messengers, passengers, and voyagers that rely on interlinked systems of vast currents, invisible ocean walls, and underwater waterfalls.