Water is our most primary natural resource. Yet, in some places, we are beginning to run out of water. Underground reserves that farmers could once reach only a few feet deep are now so low that a hole drilled half a mile down cannot find water. The great rivers we first heard about in geography lessons—strong blue lines on our atlas maps stretching all the way from mountains to the oceans—are running dry. In the real world, the blue lines have sometimes given way to desert. The Nile in Egypt, the Ganges in India and Bangladesh, the Indus in Pakistan, the Yellow River in China, and the Colorado in the United States are among the rivers that no longer always make it to the sea. Nature’s water cycle is not faltering. Our demands on it are increasing so much that, we are exhausting our water sources.