From Cnut to D-Day, the history and science of the unceasing tide is explored for the first time. Half of the world's population lives in coastal regions lapped by tidal waters. Yet how little most of us know about the tide - a key force on our planet that has altered the course of history and will transform our future. Our ability to predict and understand the tide depends on centuries of science, from the observations of Aristotle and the theories of Newton to today's supercomputer calculations. This story is punctuated here by notable tidal episodes in history, from Caesar's thwarted invasion of Britain to the catastrophic flooding of Venice and interwoven with a rich folklore that continues to inspire art and literature today.