A water layer of the Earth penetrating the atmosphere and the lithosphere. Hydrosphere covers surface, underground and atmospheric water in all states: liquid, solid and gaseous. It includes all types of natural waters: oceans, seas, rivers, lakes, glaciers, groundwater, atmospheric water, etc.
Waters of the hydrosphere are in constant movement (called the hydrological cycle) due to the solar radiation and gravitational forces.
Water resources of the hydrosphere are constant and estimated at 1.5 billion km3 of water. Of this, only 2.5% is fresh water, the rest being saline water.