In the Old Testament, the God of the Hebrews hovers over the primeval waters and brings forth the world from the infinite ocean. It is a motif echoed in creation myths throughout the world. In each, water is the fundamental precondition for life. Yet the extent to which water remains a scientific mystery is extraordinary, despite its prevalence and central importance on Earth. Whether one considers its role in biology, its place in the physical world (where it refuses to obey the usual rules of liquids) or its deceptively simple chemistry, there is still no complete answer to the question: what is water? This book explains what, exactly, we do and do not know about the strange character of this most essential and ubiquitous of substances.