The History of the Baltic Sea: Location, Facts, and Map
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The history of the Baltic Sea spans tens of thousands of years. The Baltic Sea is not one of the world's large or deep seas. Its area is only about 430,000 square kilometres, and its maximum depth reaches roughly 470 metres. Even so, depths of hundreds of metres are a rarity here — the average depth of the Baltic Sea is just 55 metres.
A building taller than 18 storeys, if placed on the average bottom of this sea, would poke above the surface. And even the deepest point of the Baltic Sea could not hide the needle of the Moscow television tower.
How salty is the Baltic Sea?
The water of the Baltic Sea is not especially salty. The many large rivers flowing into it sharply lower its salinity, which is far below that of the open ocean. The narrow, shallow straits leave little room for salt and fresh water to mix at depth. The Gulf of Finland is the least saline part of all, because the full-flowing Neva empties into it.
The rivers that feed the sea and the role of the Gulf of Finland
Freshwater inflow is the single greatest factor shaping the character of the Baltic Sea. Powerful rivers pour continuously into the basin, diluting the salt carried in from the Atlantic and keeping the sea brackish rather than truly marine. The Gulf of Finland illustrates this most clearly: fed by the Neva, its waters run so fresh that near the head of the gulf the sea behaves almost like a river estuary, with salinity dropping close to zero.
The history of the Baltic Sea from the Ice Age to today
The Baltic Sea is the child of the great glacier that once advanced across Europe from the Scandinavian mountains and covered most of the continent. At that time the entire Baltic basin lay beneath a layer of ice many kilometres thick. Then the glaciers began to retreat, exposing the dark surface of the land to the rays of the sun.
They also uncovered the floor of the Baltic basin, which was immediately filled by the waters of the melting glacier. This happened remarkably recently, about 13,000 years ago.
The Baltic Sea is the child of the great glacier
The melting of the ice was rapid, and the released water flooded the whole exposed bowl of the sea. This is the stage known as the Baltic Ice Lake. Its excess water spilled through southern Sweden into the North Sea, which is part of the Atlantic Ocean. In those days an even more convenient link between the vast lake and the ocean lay across central Sweden, but it was still choked by the body of the glacier.
When the glacier finally left central Sweden, that passage opened, and the level of the freshwater lake fell quickly until it matched the level of the ocean. Yet even once this balance was reached, the outflow of fresh water into the ocean did not stop, because the sea kept receiving meltwater from the retreating glacier. This current "out of the sea," however, ran only in the upper part of the connecting strait.
In the lower part of the strait a counter-current set in: heavy salt water from the ocean flowed into the fresh sea. The freshwater glacial lake thus became a salt sea — the Yoldia Sea, the Baltic's first saline stage, some 10,000 years ago. Its level stood about fifty metres below today's. The Danish straits had not yet opened, and one could walk to the Scandinavian Peninsula from Denmark across the future island of Gotland without getting one's boots wet.
This first sea was very short-lived, lasting only 600 to 700 years. Uplift of the earth's crust severed its links to the ocean, and the sea reverted to a lake once more.
The Ancylus Lake
The next stage in the formation of the Baltic Sea was the Ancylus Lake. Numerous rivers flowed into it, so it quickly lost its salinity and became fresh again. In its sediments geologists found a freshwater mollusc, Ancylus, which gave the lake its name.
The Ancylus Lake, too, was no long-lived feature of the planet: after roughly 1,000 years its fresh waters again began to overflow across central Sweden into the Atlantic Ocean. A passage also opened through the Danish straits, and once more a strong counter-current set in against the lake's fresh water in the lower part of the straits.
The Littorina Sea: the second saline sea
The counter-current was strong enough for the plant and animal life typical of the ocean's salt water to establish itself firmly. This second edition of the sea in the same place — the Littorina Sea — appeared roughly 7,000 years ago. At that time the sea was saltier than it is now, and the climate along its shores was warmer than today. The shaping of the Baltic Sea, of course, did not end there.
Modern outlines and the changes still under way
Rises of the earth's crust, fluctuations in sea level, and shifts in the sea's configuration and coastline all continued. Only 2,000 to 3,000 years ago did the Baltic Sea take on its present outline — and even that is subject to a number of trends toward change, because the earth's crust in the region of the Baltic Sea is rising continuously. This is bound to affect the shape of so shallow a sea as the Baltic.
Crustal uplift and today's sea-level shifts
Post-glacial uplift is the reason the Baltic coastline is not fixed. Freed of the enormous weight of the vanished ice sheet, the land around the northern Baltic is still slowly rebounding, lifting some shores clear of the water year by year. Because the Baltic Sea is so shallow, even modest vertical movement of the crust reshapes bays, exposes new land, and turns former inlets into inland lakes over the span of a few generations.
More than sixty years ago the great Russian poet Valery Bryusov wrote the poem "To the North Sea":
I have come to bid you farewell, sea, perhaps for many long years. Once again you wear your glittering attire, laced with foam, as ever.
The poet then lists historical figures who once stood on the shore of the North Sea — Julius Caesar, William of Orange, the artists who painted the distances of that sea. He closes by addressing the sea itself:
Pass on, O sea, unchanging through the ages that devour us...
One gets the impression that for this clever and deeply educated poet the sea was a kind of symbol of permanence and constancy. Today, however, we understand well that such permanence is highly relative — that only by measuring the life of the sea against the life of a single human being can one speak of the sea as unchanging.
Even within the memory of a few human generations, seas prove not to be unchanging at all. Seas and lakes vanish and appear just as islands and continents do.
The Baltic Sea as a historical region
Beyond its geology, the Baltic Sea is also a historical region — a framework historians use to study the lands ringed by its waters as a single connected space rather than a set of separate nations. The German historian Michael North, whose work for Cambridge University Press treats the Baltic as a shared arena of exchange, is among those who read the sea less as a barrier than as a highway binding its peoples together.
How the concept of the Baltic Sea Region was built
The idea of a Baltic Sea Region is a constructed historical concept, not a natural given. Scholars such as Matti Klinge, Kristian Gerner, Klas-Göran Karlsson and Ulf Zander have shown how the region was imagined and reimagined over time, and the political scientist Olle Wæver analysed how "the Baltic" could be assembled and dismantled as a category depending on the politics of the moment. The Iron Curtain of the Cold War cut the sea in two, and the enthusiasm for Baltic regionalism that followed 1989 gave the concept fresh life, drawing Sweden, Finland, Germany, Poland and Russia/USSR into renewed dialogue across the water.
The spread of Christianity and the Reformation around the Baltic
Religion was one of the strongest forces knitting the Baltic lands together. Medieval colonization carried Christianity north and east across the sea, and the later Reformation spread rapidly along the same trading and cultural networks, reshaping Sweden, Finland, the Pomerania coast and the German ports. These currents of belief moved with merchants and settlers, showing the Baltic Sea acting as a connecting link rather than a moat.
German and Russian–Soviet belonging in the Baltic Sea Region
Whether Germany and Russia/USSR truly "belong" to the Baltic Sea Region has long been contested. Both powers held long coastlines and major stakes in the sea, yet each has also been cast as an outsider by nationalist narratives that preferred a purely Nordic or purely local Baltic. Historians such as David Kirby have traced how the great powers were woven into the sea's economy and politics while remaining ambiguous members of the regional community.
The Danzig Corridor and interwar geopolitics
The Danzig Corridor made the Baltic a flashpoint of twentieth-century geopolitics. The Versailles Treaty granted a reborn Poland access to the sea through a strip of former German territory, splitting Germany from East Prussia and turning maritime access into a bitter dispute. Polish geographers and officials such as Stanisław Srokowski, together with the Baltic Institute, worked to justify and secure Poland's place on the coast, while ports like Swinoujscie and later Świnoujście / Swinoujscie became emblems of the country's contested reach to the sea.
Contested Baltic identities and the political use of history
Baltic identities have repeatedly been instrumentalized for political ends. Polish maritime tradition and the long debate over sea access, examined by scholars including Janusz Tazbir and the economic historian Marian Małowist, show how national feeling was mobilized around the coastline. The tension between nationalist and regionalist readings of the past runs throughout Baltic history, with each generation deciding whether to tell the story as a contest of nations or as a shared regional experience.
Historiography: studying connections and exchanges across the region
Modern historiography of the Baltic Sea increasingly favours transfers, networks and connections over isolated national stories. This approach owes much to Fernand Braudel, whose model of a sea as a unit of history reframed how regional seas are written about. Studying trade routes, migration and cultural exchange across the water lets historians recover a regional history beyond national narratives — the Baltic as a web of relationships rather than a line dividing rival states.
Comparing the Baltic Sea with other seas
The Baltic Sea makes most sense when set beside other regional seas. It is small, shallow and brackish where the Mediterranean Sea is deep and saline, and it is enclosed like the Black Sea, the Red Sea, the Sea of Japan or the South China Sea, each of which has framed the history of the peoples around it. Comparing these basins highlights how a single body of water can serve at once as a moat dividing neighbours and a connecting link binding them together.
| Sea | Character | Historical role |
|---|---|---|
| Baltic Sea | Shallow, brackish, nearly enclosed | Trade and cultural exchange among Sweden, Finland, Poland, Germany, Russia |
| Mediterranean Sea | Deep, saline, semi-enclosed | Braudel's classic connecting sea of many civilisations |
| Black Sea | Enclosed, low-salinity deep layers | Crossroads of Europe and Asia |
| South China Sea / Sea of Japan | Marginal Pacific seas | Contested maritime spaces and trade routes |
Studying the history of oceans and seas
The history of the Baltic Sea belongs to the broader field of oceanic history, which studies the world's waters as active agents in human affairs. The collaborative volume Oceanic Histories, edited by David Armitage of Harvard University, Alison Bashford of the University of New South Wales and Sujit Sivasundaram of the University of Cambridge and published by Cambridge University Press, places regional seas like the Baltic alongside the great oceans in a single comparative framework. Maritime and regional-sea studies of this kind treat coasts, currents and crossings as the connective tissue of history — the same lens that turns the geological story of the Baltic into a human one.
