Baltic Sea, arm of the Atlantic Ocean, c.163,000 sq mi (422,170 sq km), including the Kattegat strait, its northwestern extension. The Øresund, Store Bælt, and Lille Bælt connect the Baltic Sea with the Kattegat and Skagerrak straits, which lead to the North Sea; the Kiel Canal, across the Jutland peninsula, is a more direct connection with the North Sea. The Baltic Sea is connected with the White Sea by the White Sea–Baltic Canal, and with the Volga River by the Volga-Baltic Waterway. The Gulf of Bothnia, the Gulf of Finland, and the Gulf of Riga are the chief arms of the Baltic Sea. Of the many islands in the sea, the principal ones are Sjælland, Fyn, Lolland, Falster, and Bornholm (Denmark); Öland and Gotland (Sweden); the Åland Islands (Finland); Saaremaa (Estonia); and Rügen (Germany). Most of the Baltic is shallow, and its tides are less pronounced than those of the North Sea. The salinity of the sea is reduced by the many rivers that enter it (the Oder, Vistula, Dvina, Tornälven, Umeälv, Angermanälven, and Dalälven), and parts of the sea freeze over in winter. The Baltic was frequented from ancient times, especially because of the amber found along the coast. In the late Middle Ages commerce on the Baltic was dominated by the Hanseatic League. Copenhagen, Szczecin, Gdańsk, Riga, St. Petersburg, Helsinki, and Stockholm are the chief ports. Heavy industrialization of the countries bordering the Baltic in the 20th cent. has resulted in the massive environmental degradation of the sea and the endangerment of many of its wildlife species. This problem began to be addressed in the 1990s.