Only 3% of all water on Earth is fresh water. Most fresh water is frozen in ice sheets and glaciers (76%), with some stored underground as groundwater (23%) or on the surface (1%). Water stored underground is found in aquifers, which are saturated zones where the pores between soil and rock particles are filled with water. The top of the saturated zone is called the water table, which rises and falls depending on water levels. Wetlands are important ecosystems that help store and filter water as it moves through the water cycle and watersheds. They reduce flood risks and improve water quality by trapping pollutants.