The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by the countries of Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden, and the North and Central European Plain regions. It is the world's largest brackish water basin.
The sea stretches from 53°N to 66°N latitude and from 10°E to 30°E longitude. It is a shelf sea and marginal sea of the Atlantic with limited water exchange between the two, making it an inland sea. The Baltic Sea drains through the Danish Straits into the Kattegat by way of the Øresund, Great Belt and Little Belt. It includes the Gulf of Bothnia (divided into the Bothnian Bay and the Bothnian Sea), the Gulf of Finland, the Gulf of Riga and the Bay of Gdańsk.
The "Baltic Proper" is bordered on its northern edge, at latitude 60°N, by Åland and the Gulf of Bothnia, on its northeastern edge by the Gulf of Finland, on its eastern edge by the Gulf of Riga, and in the west by the Swedish part of the southern Scandinavian Peninsula.
The Baltic Sea is connected by artificial waterways to the White Sea via the White Sea–Baltic Canal and to the German Bight of the North Sea via the Kiel Canal.
Definitions
Administration
The Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea Area includes the Baltic Sea and the Kattegat, without calling Kattegat a part of the Baltic Sea, "For the purposes of this Convention the 'Baltic Sea Area' shall be the Baltic Sea and the Entrance to the Baltic Sea, bounded by the parallel of the Skaw in the Skagerrak at 57°44.43'N."
Traffic history
Historically, the Kingdom of Denmark collected Sound Dues from ships at the border between the ocean and the land-locked Baltic Sea, in tandem: in the Øresund at Kronborg castle near Helsingør; in the Great Belt at Nyborg; and in the Little Belt at its narrowest part then Fredericia, after that stronghold was built. The narrowest part of Little Belt is the "Middelfart Sund" near Middelfart.
Oceanography
Geographers widely agree that the preferred physical border between the Baltic and North Seas is the Langelandsbælt (the southern part of the Great Belt strait near Langeland) and the Drogden-Sill strait. The Drogden Sill is situated north of Køge Bugt and connects Dragør in the south of Copenhagen to Malmö; it is used by the Øresund Bridge, including the Drogden Tunnel. By this definition, the Danish Straits is part of the entrance, but the Bay of Mecklenburg and the Bay of Kiel are parts of the Baltic Sea.
Another usual border is the line between Falsterbo, Sweden, and Stevns Klint, Denmark, as this is the southern border of Øresund. It is also the border between the shallow southern Øresund (with a typical depth of 5–10 meters only) and notably deeper water.
Hydrography and biology
Drogden Sill (depth of 7 m (23 ft)) sets a limit to Øresund and Darss Sill (depth of 18 m (59 ft)), and a limit to the Belt Sea. The shallow sills are obstacles to the flow of heavy salt water from the Kattegat into the basins around Bornholm and Gotland.
The Kattegat and the southwestern Baltic Sea are well oxygenated and have a rich biology. The remainder of the Sea is brackish, poor in oxygen, and in species. Thus, statistically, the more of the entrance that is included in its definition, the healthier the Baltic appears; conversely, the more narrowly it is defined, the more endangered its biology appears.
Etymology and nomenclature
The precise origin of the name is unknown. Tacitus called it the Suebic Sea, Latin: Mare Suebicum after the Germanic people of the Suebi, and Ptolemy Sarmatian Ocean after the Sarmatians, but the first to name it the Baltic Sea (Medieval Latin: Mare Balticum) was the eleventh-century German chronicler Adam of Bremen. It might be connected to the Germanic word belt, a name used for two of the Danish Straits, the Belts, while others claim it to be directly derived from the source of the Germanic word, Latin balteus "belt". Adam of Bremen himself compared the sea with a belt, stating that it is so named because it stretches through the land as a belt (Balticus, eo quod in modum baltei longo tractu per Scithicas regiones tendatur usque in Greciam).
He might also have been influenced by the name of a legendary island mentioned in the Natural History of Pliny the Elder. Pliny mentions an island named Baltia (or Balcia) with reference to accounts of Pytheas and Xenophon. It is possible that Pliny refers to an island named Basilia ("the royal") in On the Ocean by Pytheas. Baltia also might be derived from "belt", and therein mean "near belt of sea, strait".
Others have suggested that the name of the island originates from the Proto-Indo-European root *bʰel meaning "white, fair", which may echo the naming of seas after colours relating to the cardinal points (as per Black Sea and Red Sea). This *bʰel root and basic meaning were retained in Lithuanian (as baltas), Latvian (as balts) and Slavic (as bely). On this basis, a related hypothesis holds that the name originated from this Indo-European root via a Baltic language such as Lithuanian. Another explanation is that, while derived from the aforementioned root, the name of the sea is related to names for various forms of water and related substances in several European languages, that might have been originally associated with colors found in swamps (compare Proto-Slavic *bolto "swamp"). Yet another explanation is that the name originally meant "enclosed sea, bay" as opposed to open sea.
In the Middle Ages the sea was known by a variety of names. The name Baltic Sea became dominant after 1600. Usage of Baltic and similar terms to denote the region east of the sea started only in the 19th century.
Name in other languages
The Baltic Sea was known in ancient Latin language sources as Mare Suebicum or even Mare Germanicum. Older native names in languages that used to be spoken on the shores of the sea or near it usually indicate the geographical location of the sea (in Germanic languages), or its size in relation to smaller gulfs (in Old Latvian), or tribes associated with it (in Old Russian the sea was known as the Varangian Sea). In modern languages, it is known by the equivalents of "East Sea", "West Sea", or "Baltic Sea" in different languages:
"Baltic Sea" is used in Modern English; in the Baltic languages Latvian (Baltijas jūra; in Old Latvian it was referred to as "the Big Sea", while the present day Gulf of Riga was referred to as "the Little Sea") and Lithuanian (Baltijos jūra); in Latin (Mare Balticum) and the Romance languages French (Mer Baltique), Italian (Mar Baltico), Portuguese (Mar Báltico), Romanian (Marea Baltică) and Spanish (Mar Báltico); in Greek (Βαλτική Θάλασσα Valtikí Thálassa); in Albanian (Deti Balltik); in Welsh (Môr Baltig); in the Slavic languages Polish (Morze Bałtyckie or Bałtyk), Czech (Baltské moře or Balt), Slovene (Baltsko morje), Bulgarian (Балтийско море Baltijsko More), Kashubian (Bôłt), Macedonian (Балтичко Море Baltičko More), Ukrainian (Балтійське море Baltijs′ke More), Belarusian (Балтыйскае мора Baltyjskaje Mora), Russian (Балтийское море Baltiyskoye More) and Serbo-Croatian (Baltičko more / Балтичко море); in Hungarian (Balti-tenger).
In Germanic languages, except English, "East Sea" is used, as in Afrikaans (Oossee), Danish (Østersøen [ˈøstɐˌsøˀn̩]), Dutch (Oostzee), German (Ostsee), Low German (Oostsee), Icelandic and Faroese (Eystrasalt), Norwegian (Bokmål: Østersjøen [ˈø̂stəˌʂøːn]; Nynorsk: Austersjøen), and Swedish (Östersjön). In Old English it was known as Ostsǣ, which does not however mean 'east sea' and may be related to a people known in the same work as the Osti. Also in Hungarian the former name was Keleti-tenger ("East-sea", due to German influence). In addition, Finnish, a Finnic language, uses the term Itämeri "East Sea", possibly a calque from a Germanic language. As the Baltic is not particularly eastward in relation to Finland, the use of this term may be a leftover from the period of Swedish rule.
In another Finnic language, Estonian, it is called the "West Sea" (Läänemeri), with the correct geography relative to Estonia (the sea being to its west). In South Estonian, it has the meaning of both "West Sea" and "Evening Sea" (Õdagumeri). In the endangered Livonian language of Latvia, it (and sometimes the Irbe Strait as well) is called the "Large Sea" (Sūŗ meŗ or Sūr meŗ).
History
Classical world
At the time of the Roman Empire, the Baltic Sea was known as the Mare Suebicum or Mare Sarmaticum. 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. Jordanes called it the Germanic Sea in his work, the Getica.
Middle Ages
In the early Middle Ages, Norse (Scandinavian) merchants built a trade empire all around the Baltic. Later, the Norse fought for control of the Baltic against Wendish tribes dwelling on the southern shore. The Norse also used the rivers of Russia for trade routes, finding their way eventually to the Black Sea and southern Russia. This Norse-dominated period is referred to as the Viking Age.
Since the Viking Age, the Scandinavians have referred to the Baltic Sea as Austmarr ("Eastern Sea"). "Eastern Sea", appears in the Heimskringla and Eystra salt appears in Sörla þáttr. 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. Another form of the name, "Grandvik", attested in at least one English translation of Gesta Danorum, is likely to be a misspelling.
In addition to fish the sea also provides amber, especially from its southern shores within today's borders of Poland, Russia and Lithuania. First mentions of amber deposits on the South Coast of the Baltic Sea date back to the 12th century. The bordering countries have also traditionally exported lumber, wood tar, flax, hemp and furs by ship across the Baltic. Sweden had from early medieval times exported iron and silver mined there, while Poland had and still has extensive salt mines. Thus, the Baltic Sea has long been crossed by much merchant shipping.
The lands on the Baltic's eastern shore were among the last in Europe to be converted to Christianity. This finally happened during the Northern Crusades: Finland in the twelfth century by Swedes, and what are now Estonia and Latvia in the early thirteenth century by Danes and Germans (Livonian Brothers of the Sword). The Teutonic Order gained control over parts of the southern and eastern shore of the Baltic Sea, where they set up their monastic state. Lithuania was the last European state to convert to Christianity.
An arena of conflict
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.
Starting in the 11th century, the southern and eastern shores of the Baltic were settled by migrants mainly from Germany, a movement called the Ostsiedlung ("east settling"). Other settlers were from the Netherlands, Denmark, and Scotland. The Polabian Slavs were gradually assimilated by the Germans. Denmark gradually gained control over a large portion of the Baltic coast, until it lost many of its possessions after being defeated in the 1227 Battle of Bornhöved.
Between the 13th and 17th centuries the Baltic Sea was, depending on the period, dominated by the Netherlands, Germany, Sweden and Russia. In the 13th to 16th centuries, the strongest economic force in Northern Europe was the Hanseatic League, a federation of merchant cities around the Baltic Sea and the North Sea. In the sixteenth and early seventeenth centuries, Poland, Denmark, and Sweden fought wars for Dominium maris baltici ("Lordship over the Baltic Sea"). Eventually, it was Sweden that virtually encompassed the Baltic Sea. In Sweden, the sea was then referred to as Mare Nostrum Balticum ("Our Baltic Sea"). The goal of Swedish warfare during the 17th century was to make the Baltic Sea an all-Swedish sea (Ett Svenskt innanhav), something that was accomplished except the part between Riga in Latvia and Stettin in Pomerania. However, the Dutch dominated the Baltic trade in the seventeenth century.
In the eighteenth century, Russia and Prussia became the leading powers over the sea. Sweden's defeat in the Great Northern War brought Russia to the eastern coast. Russia became and remained a dominating power in the Baltic. 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. There was much trading not just within the Baltic region but also with the North Sea region, especially eastern England and the Netherlands: their fleets needed the Baltic timber, tar, flax, and hemp.
During the Crimean War, a joint British and French fleet attacked the Russian fortresses in the Baltic; the case is also known as the Åland War. They bombarded Sveaborg, which guards Helsinki; and Kronstadt, which guards Saint Petersburg; and they destroyed Bomarsund in Åland. After the unification of Germany in 1871, the whole southern coast became German. World War I was partly fought in the Baltic Sea. After 1920 Poland was granted access to the Baltic Sea at the expense of Germany by the Polish Corridor and enlarged the port of Gdynia in rivalry with the port of the Free City of Danzig.
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 1945, the Baltic Sea became a mass grave for retreating soldiers and refugees on torpedoed troop transports. The sinking of the Wilhelm Gustloff remains the worst maritime disaster in history, killing (very roughly) 9,000 people. 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.
Since World War II
Ammunition dumping
Since the end of World War II, various nations, including the Soviet Union, the United Kingdom and the United States have disposed of chemical weapons in the Baltic Sea, raising concerns of environmental contamination. Today, fishermen occasionally find some of these materials: the most recent available report from the Helsinki Commission notes that four small scale catches of chemical munitions representing approximately 105 kg (231 lb) of material were reported in 2005. This is a reduction from the 25 incidents representing 1,110 kg (2,450 lb) of material in 2003. Until now, the U.S. Government refuses to disclose the exact coordinates of the wreck sites. Deteriorating bottles leak mustard gas and other substances, thus slowly poisoning a substantial part of the Baltic Sea.
In addition to chemicals, tons of German ammunition were dumped into the Baltic after the war at the behest of the Allies, who wanted to ensure that they would not start another war. By 2025, these were leaking contaminants into the water and the German government was piloting solutions.
Territorial changes
After 1945, the German population was expelled from all areas east of the Oder-Neisse line, making room for new Polish and Russian settlement. Poland gained most of the southern shore. The Soviet Union gained another access to the Baltic with the Kaliningrad Oblast, that had been part of German-settled East Prussia. The Baltic states on the eastern shore were annexed by the Soviet Union. The Baltic then separated opposing military blocs: NATO and the Warsaw Pact. During the Cold War the Baltic Sea remained on the periphery of the main axis of confrontation between these two blocs. Neutral Sweden developed incident weapons to defend its territorial waters after the Swedish submarine incidents. This border status restricted trade and travel. It ended only after the collapse of the Communist regimes in Central and Eastern Europe in the late 1980s.
Finland and Sweden joined NATO in 2023 and 2024, respectively, making the Baltic Sea almost entirely surrounded by the alliance's members, leading some commentators to label the sea a "NATO lake". However, the legal status of the sea has not changed and it is still open to all nations. Such an arrangement has also existed for the European Union (EU) since May 2004 following the accession of the Baltic states and Poland. The remaining non-NATO and non-EU shore areas are Russian: the Saint Petersburg area and the Kaliningrad Oblast exclave.
Infrastructure
The Baltic Sea today is of significant economic and security importance due to its dense network of submarine cables, energy pipelines, ports and offshore energy platforms. In recent years, there have been a number of incidents of sabotage in the Baltic Sea, resulting in damage to critical infrastructures. The most notable incidents include the Nord Stream pipelines sabotage in 2022, where a series of underwater explosions destroyed both Nord Stream 1 and 2. In 2023, there was another incident involving the Balticconnector gas pipeline and a nearby data cable, which were damaged by the Hong Kong-flagged container ship NewNew Polar Bear.
Other significant incidents include the recent damage to several undersea communication cables. The most recent incident of relevance was the rupture of the Estlink 2 cable in late 2024. It is suspected that the oil tanker Eagle S, believed to be part of a Russian shadow fleet, is responsible.
These events followed a series of responses from both NATO and the EU. In response, NATO Baltic Sea states have increased their naval presence in the Baltic Sea, and the NATO operation Baltic Sentry was established. Simultaneously, the EU has implemented a series of measures designed to enhance the protection of critical maritime infrastructure. The EU has also underscored the commitment to strengthening cooperation with NATO.
Storms and storm floods
Winter storms begin arriving in the region during October. These have caused numerous shipwrecks, and contributed to the extreme difficulties of rescuing passengers of the ferry MS Estonia en route from Tallinn, Estonia, to Stockholm, Sweden, in September 1994, which claimed the lives of 852 people. 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.
Storm surge floods are generally taken to occur when the water level is more than one metre above normal. In Warnemünde about 110 floods occurred from 1950 to 2000, an average of just over two per year.
Historic flood events were the All Saints' Flood of 1304 and other floods in the years 1320, 1449, 1625, 1694, 1784 and 1825. Little is known of their extent. From 1872, there exist regular and reliable records of water levels in the Baltic Sea. The highest was the flood of 1872 when the water was an average of 2.43 m (8 ft 0 in) above sea level at Warnemünde and a maximum of 2.83 m (9 ft 3 in) above sea level in Warnemünde. In the last very heavy floods the average water levels reached 1.88 m (6 ft 2 in) above sea level in 1904, 1.89 m (6 ft 2 in) in 1913, 1.73 m (5 ft 8 in) in January 1954, 1.68 m (5 ft 6 in) on 2–4 November 1995 and 1.65 m (5 ft 5 in) on 21 February 2002.
Geography
Geophysical data
An arm of the North Atlantic Ocean, the Baltic Sea is enclosed by Sweden and Denmark to the west, Finland to the northeast, and the Baltic countries to the southeast.
It is about 1,600 km (990 mi) long, an average of 193 km (120 mi) wide, and an average of 55 metres (180 ft) deep. The maximum depth is 459 m (1,506 ft) which is on the Swedish side of the centre. The surface area is about 349,644 km2 (134,998 sq mi) and the volume is about 20,000 km3 (4,800 cu mi). The periphery amounts to about 8,000 km (5,000 mi) of coastline.
The Baltic Sea is one of the largest brackish inland seas by area, and occupies a basin (a Zungenbecken) formed by glacial erosion during the last few ice ages.