The Baltic Sea is an inland sea. Hundreds of rivers flow into the Baltic Sea, filling it with fresh water. However, the Baltic Sea also receives high-salt seawater from the North Sea. As these two water types are combined, the result is a water type called brackish water. Brackish water is low in salt, with a concentration of a few promilles (thousandths) that is quite low when compared to the salt concentration of 3,5 percent found in the major oceans. 

The Baltic sea is also quite shallow. As a result, it contains relatively little water compared to other seas. 

The Baltic Sea is exceptional in many respects. For one, it is a relatively young sea, being formed after the last ice age some 10 000 years ago.

The Baltic Sea is also special due to the fact that it is the largest shallow brackish water pool of its type in the world. In addition to this, the plants and animals of the Baltic Sea are unique as they have had to adapt to the sea freezing over during most winters.

The catchment area of the Baltic Sea (the area from which water flows into the Baltic) is home to over a hundred million people. As a result, organisms also have to adapt to the changes brought into their environment by human activity.