Inspired by Homer's description of the ebb and flow of the "wine dark sea," the ancient Greeks conceived a back-and-forth movement of blood. That false notion, perpetuated by the influential Roman physician Galen, prevailed for fifteen hundred years until William Harvey proved that blood circulates: the heart pumps blood in one direction through the arteries and it returns through the veins. Harvey's discovery revolutionized the life sciences by making possible an entirely new quantitative understanding of the cardiovascular system, a way of thinking on which many of our lifesaving medical interventions today depend. In The Wine-Dark Sea Within, cardiologist Dhun Sethna argues that Harvey's revelation inaugurated modern medicine and paved the way for groundbreaking advances from intravenous therapy, cardiac imaging, and stent insertions to bypass surgery, dialysis, and heart-lung machines. Weaving together three thousand years of global history, following bitter feuds and epic alliances, tragic failures and extraordinary advancements, this is a provocative history by a fresh voice in popular science.