More than four billion years ago, the most important event in Earth's history took place: the ocean was born. It completely transformed the planet, creating a watery oasis that gave rise to the air that organisms consume, the climate, and a stunning array of life. The ocean is what transformed Earth into the livable, blue planet it is today. This episode tells the story of the ocean's turbulent beginnings and its successive incarnations. Part of the "One Ocean" series.