Water on Earth is constantly moving in different forms. It is found in the clouds in the sky, waves in the ocean, flowing rivers and runoff streams, underground aquifers, and even frozen glaciers. This continuous movement on, above, and under the Earth's surface that transforms water into solid, liquid, and gas forms is known as the water cycle, a natural process that was not fully understood until the 19^th century. In The Water Cycle, read about how water moves through this cycle and how scientists came to understand it, as well as how treated wastewater is used to conserve our dwindling freshwater resources.