In the water cycle, water evaporates from the water system (oceans, lakes, rivers, etc.) to the atmosphere, condenses into clouds, moves inland, precipitates onto land and finally flows back into the sea. 

Evaporating water is pure and fresh. When the water rains back down to the ground, different salts begin to dissolve into it. These salts are then carried into the seas with the water. 

When water evaporates from the ocean to the atmosphere, the salts remain in the ocean instead of evaporating with the water molecules. Over the course of millions of years, different salts have accumulated in the planet's seas and oceans. As a result of this, these areas of water have turned saline. The most common type of salt found in seawater is sodium chloride, but other types of salts can also be found in it. 

Only three percent of Earth's water is non-saline fresh water. Most of it is stored in glacial ice (70 % of the planet's fresh water, meaning under 2 % of its total water). Fresh water is also found under the ground in the form of groundwater. 

Only under one percent of fresh water appears as surface water in lakes, rivers, and ponds. Life is mostly found in unfrozen, liquid water, but even glaciers are inhabited by some simple organisms, such as unicellular algae.

Salts change the properties of the water into which they dissolve. The salt concentration of the oceans is approximately 3,5 %. Brackish water, which has a salt concentration of under one percent, is found in the Baltic Sea and other similar areas.