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Baltic Sea
Northern Europe's brackish, ice-fringed inland sea
The Baltic is the world's largest expanse of brackish water — neither truly fresh nor fully salt — a shallow, cold sea that reaches deep into northern Europe. Its tea-coloured waters, low salinity, and winter ice set it apart from every other sea, and its shores have been the highway of Vikings, the trading network of the Hanseatic League, and the contested frontier of empires. Amber washed from its floor has been treasured since the Stone Age, carried south along trade routes that crossed the continent.
Covering roughly 349,000 square kilometres but averaging only about 55 metres deep, the Baltic is a young sea, scooped out by Ice Age glaciers and still slowly rising as the land rebounds. Its deepest point, the Landsort Deep, reaches 459 metres. Hundreds of rivers pour fresh water in, while only a trickle of salt water enters through the narrow Danish straits, leaving the sea stratified and slow to renew. This sluggish exchange makes it acutely vulnerable to pollution, and large areas of its seabed have become oxygen-starved dead zones.
Nine modern nations ring the Baltic, and its history is inseparable from theirs. Medieval merchant cities from Lübeck to Novgorod grew rich on its trade in furs, fish, grain, and amber — later it became the strategic theatre of Swedish, Russian, and German ambitions. During the Cold War it was an iron-curtain frontier. Today it is one of the most intensively studied and protected seas in the world, even as eutrophication and a warming climate reshape its fragile, low-salt ecosystem.