The Baltic Sea is a Mediterranean-style sea located in Northern Europe, surrounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It is unique for being the world's largest body of 'brackish' water—a mixture of fresh water from hundreds of rivers and salt water from the North Sea. Because the connection to the ocean is so narrow (the Danish Straits), it takes about 30 years for all the water in the Baltic to be completely exchanged. This low salinity means parts of the sea freeze over every winter, which is rare for large salt-water bodies. The Baltic has played a central role in history, particularly during the era of the Hanseatic League trade network. Today, it is an ecologically sensitive area, suffering from eutrophication—a process where excess nutrients lead to massive algae blooms that deplete oxygen in the water.