The features of the Baltic Sea make it unique The Baltic Sea is the youngest in the world. It is surrounded by continents on almost every side and the connection to the sea is both narrow and shallow. Every year, the surface of the Baltic Sea is at least partially covered by ice. Compared to many other seas, the Baltic Sea is low in salinity and quite shallow. These features make it a globally unique marine area. The surface area of the Baltic Sea is around 392,000 squarekilometres, which is only a fraction of the Atlantic Ocean’s surface area, but still more than the surface area of Finland, for example. The Baltic Sea in numbers The Baltic Sea is 54 metres deep on average. In comparison, the average depth of the Mediterranean Sea is around 1,500 metres and 4,000 metres in the great oceans. The Baltic Sea in numbers The salinity of the Baltic Sea varies. The salinity in the Danish straits is 20‰, but only 2‰ in the Bothnian Bay. The average salinity of the Baltic Sea is 7‰. Salinity, temperature and stratification The history the Baltic Sea is divided into several stages. For example, 10,700 years ago, the Baltic Sea was a lake. 20,000 years ago, the Baltic Sea was completely covered by a continental glacier. Evolution of the Baltic Sea Share this page Share on Twitter Share on Facebook The Baltic Sea is small, shallow and surrounded by several states