The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden, and the North and Central European Plain.

The origin of the latter name is speculative and it was adopted into Slavic and Finnic languages spoken around the sea, very likely due to the role of Medieval Latin in cartography.

Tacitus in his AD 98 Agricola and Germania described the Mare Suebicum, named for the Suebi tribe, during the spring months, as a brackish sea where the ice broke apart and chunks floated about.

The Suebi eventually migrated southwest to temporarily reside in the Rhineland area of modern Germany, where their name survives in the historic region known as Swabia.

Saxo Grammaticus recorded in Gesta Danorum an older name, Gandvik, -vik being Old Norse for "bay", which implies that the Vikings correctly regarded it as an inlet of the sea.

[citation needed] In addition to fish the sea also provides amber, especially from its southern shores within today's borders of Poland, Russia and Lithuania.

The Teutonic Order gained control over parts of the southern and eastern shore of the Baltic Sea, where they set up their monastic state.

[citation needed] In the period between the 8th and 14th centuries, there was much piracy in the Baltic from the coasts of Pomerania and Prussia, and the Victual Brothers held Gotland.

In the sixteenth and early seventeenth centuries, Poland, Denmark, and Sweden fought wars for Dominium maris baltici ("Lordship over the Baltic Sea").

Russia's Peter the Great saw the strategic importance of the Baltic and decided to found his new capital, Saint Petersburg, at the mouth of the Neva river at the east end of the Gulf of Finland.

[citation needed] After the Nazis' rise to power, Germany reclaimed the Memelland and after the outbreak of the Eastern Front (World War II) occupied the Baltic states.

In 2005, a Russian group of scientists found over five thousand airplane wrecks, sunken warships, and other material, mainly from World War II, on the bottom of the sea.

[citation needed] After 1945, the German population was expelled from all areas east of the Oder-Neisse line, making room for new Polish and Russian settlement.

Older, wood-based shipwrecks such as the Vasa tend to remain well-preserved, as the Baltic's cold and brackish water does not suit the shipworm.

Freezing begins in the northern extremities of the Gulf of Bothnia typically in the middle of November, reaching the open waters of the Bothnian Bay in early January.

Due to the often fluctuating winter temperatures between above and below freezing, the saltwater ice of the Baltic Sea can be treacherous and hazardous to walk on, in particular in comparison to the more stable fresh water-ice sheets in the interior lakes.

It mixes very slowly with the upper waters, resulting in a salinity gradient from top to bottom, with most of the saltwater remaining below 40 to 70 m (130 to 230 ft) deep.

Quite violent, sudden storms sweep the surface ten or more times a year, due to large transient temperature differences and a long reach of the wind.

This creates a vertical stratification of the water column, a halocline, that represents a barrier to the exchange of oxygen and nutrients, and fosters completely separate maritime environments.

Further east and north the salinity at the bottom is consistently lower, being the lowest in Bothnian Bay (depths greater than 120 m or 390 ft) where it is slightly below 0.4%, or only marginally higher than the surface in the same region.

Geological surveys show that before the Pleistocene, instead of the Baltic Sea, there was a wide plain around a great river that paleontologists call the Eridanos.

There are a number of named and dated stages in the evolution of the Baltic Sea:[59] The land is still emerging isostatically from its depressed state, which was caused by the weight of ice during the last glaciation.

[39] For example, the common starfish and shore crab, two species that are very widespread along European coasts, are both unable to cope with the significantly lower salinity.

[86] Satellite images taken in July 2010 revealed a massive algal bloom covering 377,000 square kilometres (146,000 sq mi) in the Baltic Sea.

[91]: 458  High confidence future projections include: air temperature warming, more heavy precipitation episodes, less snow with less perifrost and glacial ice mass in northern catchment areas, more mild winters, raised mean water temperature with more marine heatwaves, intensified seasonal thermoclines without change in the thermohaline circulation, and sea level rise.

The largest shipyards are at Gdańsk, Gdynia, and Szczecin, Poland; Kiel, Germany; Karlskrona and Malmö, Sweden; Rauma, Turku, and Helsinki, Finland; Riga, Ventspils, and Liepāja, Latvia; Klaipėda, Lithuania; and Saint Petersburg, Russia.

There are several cargo and passenger ferries that operate on the Baltic Sea, such as Construction of the Fehmarn Belt Fixed Link between Denmark and Germany is due to finish in 2029.

In the light of political changes and developments in international environmental and maritime law, a new convention was signed in 1992 by all the states bordering on the Baltic Sea, and the European Community.

The present contracting parties are Denmark, Estonia, the European Community, Finland, Germany, Latvia, Lithuania, Poland, Russia, and Sweden.

The ratification instruments were deposited by the European Community, Germany, Latvia and Sweden in 1994, by Estonia and Finland in 1995, by Denmark in 1996, by Lithuania in 1997, and by Poland and Russia in November 1999.