The Baltic Sea is a semi-enclosed sea of about 415 000 km2 (Figure 2- 1). Proceeding from the northern end, it includes the Bothnian Bay and the Bothnian Sea. At the southern end of the Bothnian Sea, the island of Aland divides the Aland Sea from the Archipelago Sea. The Gulf of Finland is the eastern arm of the Baltic Sea. The central portion of the Sea, known as the Baltic Proper, includes the Eastern and Western Gotland Seas. To the east and south are the Gulf of Riga, and the Gulf of Gdansk.