The Critical Zone supports terrestrial life on Earth. It is the region above and below the Earth surface, extending from the tops of the trees down through the subsurface to the bottom of the groundwater. It is a living, breathing, constantly evolving boundary layer where rock, soil, water, air, and living organisms interact. These complex interactions regulate the natural habitat and determine the availability of life-sustaining resources, including our food production and water quality. Critical Zone scientists work to discover how this living skin is structured, evolves, and provides essential functions that sustain life. The national Critical Zone Observatory Network is made up of nine environmental observatories each located in a different climatic and geologic setting.