Over the past two generations—and especially since the pioneering 1972 UN Conference on Human Environment and then the 1977 Mar Del Plata UN Conference on Water—public attention has been drawn to the significance of water as the key to human sustenance. Always present implicitly, historically water was perceived as an ingredient for agriculture, transportation, industry, and human development. But when water was studied explicitly, it was typically by hydrologists, civil engineers, and chemists who sought to know its properties, characteristics, and potential for irrigation and power. And nearly always, visible water—that is, surface water—was the main subject of inquiry.