The hydrological cycle describes the constant movement of water between the earth and atmosphere in its liquid, solid, and gaseous states. Key components of the cycle include evaporation of water from oceans into the atmosphere, condensation of water vapor back into liquid form, and storage of water on land as ice, snow, groundwater, lakes, and rivers. The cycle has sustained life on Earth for millions of years by recycling water, though human use of water must be sustainable to avoid disrupting the cycle and causing shortages during droughts.