Baltic Sea

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, meaning it contains a mix of fresh and salt water.

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, making it almost like 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 several important areas such as 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 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 also 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 talks about the Baltic Sea and the Kattegat, but it doesn’t call Kattegat a part of the Baltic Sea. It says the "Baltic Sea Area" includes the Baltic Sea and the entrance to it, ending at a line in the Skagerrak at 57°44.43'N.

Traffic history

In the past, the Kingdom of Denmark made ships pay money called Sound Dues at three places: at Kronborg castle near Helsingør in the Øresund, at Nyborg in the Great Belt, and at its narrowest part near Fredericia in the Little Belt.

Oceanography

Experts say the border between the Baltic Sea and the North Sea is usually at the Langelandsbælt, the southern part of the Great Belt near Langeland, and the Drogden-Sill strait. The Drogden Sill is north of Køge Bugt and connects Dragør near Copenhagen to Malmö. It is where the Øresund Bridge is, including the Drogden Tunnel. With this border, the Danish Straits are part of the entrance, but the Bay of Mecklenburg and the Bay of Kiel are part of the Baltic Sea. Another common border is the line between Falsterbo in Sweden and Stevns Klint in Denmark. This line marks the edge of the shallow southern Øresund, which is only 5 to 10 meters deep, and the deeper water beyond.

Hydrography and biology

The Drogden Sill, which is only 7 meters deep, limits the flow into Øresund, and the Darss Sill, at 18 meters deep, limits the flow into the Belt Sea. These shallow areas make it hard for salty water from the Kattegat to reach places around Bornholm and Gotland.

The Kattegat and southwestern Baltic Sea have lots of oxygen and many kinds of plants and animals. The rest of the Baltic Sea has less salt, less oxygen, and fewer plants and animals. So, if you include more of the entrance in the definition of the Baltic Sea, it seems healthier. But if you define it more narrowly, it looks more at risk.

Etymology and nomenclature

The exact origin of the name "Baltic Sea" is not known. Ancient writers had different names for it. One called it the Suebic Sea after a group of people, while another used the name Sarmatian Ocean. The name Baltic Sea was first used by a German writer in the eleventh century. He may have chosen this name because the sea stretches through the land like a belt.

Other ideas about the name’s origin include links to words meaning “white” or “fair” in ancient languages, or to words for “enclosed sea” or “bay”. During the Middle Ages, the sea had many different names, but the name Baltic Sea became common after the year 1600. Today, many languages have their own names for the Baltic Sea, often based on its location or size.

History

Classical world

During the time of the Roman Empire, the Baltic Sea was called the Mare Suebicum or Mare Sarmaticum. Tacitus wrote about it in AD 98, describing it as a brackish sea where ice would break apart in spring. The name came from the Suebi tribe. Later, Jordanes called it the Germanic Sea in his work, the Getica.

Middle Ages

In the early Middle Ages, Scandinavian traders built a big trading network around the Baltic. They also fought for control of the sea with tribes living on the southern shore. The Scandinavians used rivers in Russia to travel to the Black Sea and southern Russia. This time is known as the Viking Age.

Since then, the Scandinavians have called the Baltic Sea Austmarr, meaning "Eastern Sea". The lands around the Baltic's eastern shore were some of the last in Europe to become Christian. This happened during the Northern Crusades: Finland was Christianized by Swedes in the twelfth century, and Estonia and Latvia in the early thirteenth century by Danes and Germans. The Teutonic Order controlled parts of the southern and eastern shore, and Lithuania was the last European country to adopt Christianity.

An arena of conflict

Between the 8th and 14th centuries, there was a lot of piracy around the Baltic coasts. In the 11th century, people from Germany began settling along the southern and eastern shores, a movement called the Ostsiedlung ("east settling"). Settlers also came from the Netherlands, Denmark, and Scotland.

Between the 13th and 17th centuries, the Baltic Sea was controlled by different countries at different times, including the Netherlands, Germany, Sweden, and Russia. In the 13th to 16th centuries, the Hanseatic League, a group of trading cities, was very powerful around the Baltic and the North Sea. In the sixteenth and early seventeenth centuries, Poland, Denmark, and Sweden fought for control over the sea.

In the eighteenth century, Russia and Prussia became the main powers over the Baltic Sea. Russia's Peter the Great founded his new capital, Saint Petersburg, near the Gulf of Finland. There was a lot of trading between the Baltic region and places like England and the Netherlands.

Since World War II

After World War II, many people had to leave their homes in areas east of the Oder-Neisse line. Poland took control of most of the southern shore, and the Soviet Union took over the Baltic states on the eastern shore.

Finland and Sweden joined NATO in 2023 and 2024, meaning the Baltic Sea is almost completely surrounded by NATO members. However, the sea remains open to all nations.

Storms and storm floods

Winter storms start in October and can cause big waves and shipwrecks. One famous shipwreck was the ferry MS Estonia in 1994, which had many passengers on board. The cold and brackish water of the Baltic helps preserve old wooden ships, like the Vasa.

Big floods happen when the water level rises more than one meter above normal. There have been many floods over the years, with the highest recorded in 1872.

Geography

An arm of the North Atlantic Ocean, the Baltic Sea is surrounded by Sweden, Denmark, Finland, and the Baltic countries. It stretches about 1,600 km long and averages 193 km wide, with an average depth of 55 meters. The deepest point reaches 459 meters.

The Baltic Sea is one of the world's largest brackish water bodies. It was formed by glacial erosion during the last few ice ages. The sea's surface area covers around 349,644 km², and its volume is about 20,000 km³. It has roughly 8,000 km of coastline.

Geology

Main article: Geology of the Baltic Sea

The Baltic Sea looks a bit like a riverbed, with two parts called the Gulf of Finland and the Gulf of Bothnia. Long ago, before huge glaciers shaped the land, this area was a wide plain around a big river called the Eridanos. Over time, glaciers carved out the land, turning the riverbed into the sea we see today.

The land around the Baltic Sea is still rising up after being pressed down by the weight of ice. This means the sea's area and depth are slowly getting smaller. In some places, this rise is about eight millimeters every year, and old seabed is becoming new land.

Biology

Fauna and flora

See also: List of fish in Sweden

The Baltic Sea is home to a mix of sea and freshwater animals and plants. You can find sea fish like Atlantic cod, Atlantic herring, and European flounder, as well as freshwater fish such as European perch and northern pike. In some places, freshwater species are more common, especially in the northern parts of the sea.

The blue mussel is very common and makes up most of the animal life in the sea. Some special animals live only in the Baltic Sea, like a type of brown seaweed and a special kind of flounder. There are also some older sea animals that stayed behind after the last big freeze, long ago.

Cetaceans like the harbor porpoise live in the Baltic Sea, and sometimes other bigger whales visit.

Environmental status

In 2010, a big green bloom covered a large part of the sea. This happens every summer because of extra nutrients from farms. About a quarter of the sea floor has very little oxygen, which makes it hard for most sea creatures to live there.

After World War II, old weapons were dropped into the sea. Experts say this can be dangerous for people living near the coast.

Future change

Climate change and pollution from farming and forests could greatly affect the Baltic Sea. Scientists worry the sea might change from storing carbon to releasing carbon dioxide and methane into the air. Predicting these changes is difficult because the Baltic Sea has special features, like its mix of fresh and salty water and sometimes icy winters. Future predictions with high confidence include warmer air, more heavy rain, milder winters, warmer water with more heatwaves, and rising sea levels. Other predictions exist but are less certain.

Economy

See also: Baltic Sea cruiseferries, Ports of the Baltic Sea, and List of oil and gas fields of the Baltic Sea

The Baltic Sea is very important for trade, especially for moving oil from Russia to other places. Many countries around the Baltic Sea worry about oil spills because the sea’s water doesn’t change quickly, which could be very harmful. The area also has a big tourism industry that worries about pollution, like oil in the water. ^{[citation needed]}

Many ships are built in shipyards around the Baltic Sea. Some of the biggest shipyards are in cities like Gdańsk, Gdynia, and Szczecin in Poland; Kiel in Germany; Karlskrona and Malmö in Sweden; Rauma, Turku, and Helsinki in Finland; Riga, Ventspils, and Liepāja in Latvia; Klaipėda in Lithuania; and Saint Petersburg in Russia. A big bridge connecting Denmark and Sweden was finished in the late 1990s, and a new tunnel between Denmark and Germany is set to be completed in 2029. The Baltic Sea is also becoming an important place for wind energy, with plans to have a lot of offshore wind power ready by 2030.

Ferries

There are many ferries that carry people and cargo on the Baltic Sea. Some of these ferries include:

• Birka Gotland (cruises from Stockholm to Gotland and Åland Islands)
• Destination Gotland (Gotland-mainland Sweden)
• Eckerö Line (Estonia-Finland)
• Eckerö Linjen (Sweden-Åland Islands)
• Finnlines (Finland-Germany, Finland-Sweden, Germany-Sweden, Poland-Sweden)
• Polferries (Poland-Sweden, Poland-Denmark)
• Scandlines (Denmark-Germany)
• Stena Line (Denmark-Sweden, Germany-Sweden, Latvia-Sweden, Poland-Sweden)
• Tallink and Tallink Silja (Estonia-Finland, Estonia-Sweden, Finland-Sweden)
• TT-Line (Germany-Lithuania, Germany-Sweden, Lithuania-Sweden, Poland-Sweden)
• Unity Line (Poland-Sweden)
• Viking Line (Estonia-Finland, Finland-Sweden)
• Wasaline (Finland-Sweden)

Tourism

Critical Maritime Infrastructure (CMI)

Critical maritime infrastructure includes important systems like pipelines, ports, undersea cables, and energy installations. In recent years, these systems in the Baltic Sea have become more important politically. Several incidents happened between 2022 and 2025, including damage to important pipelines and cables. These events have led to increased attention and action from groups like NATO and the European Union, as well as from countries in the area. They are working together to protect these important systems better.

The Baltic Sea has also faced challenges from what are called "hybrid threats." These are actions that happen below the level of war but can still cause big problems. They include things like secret damage and tricks with information. These threats make it hard to know who is responsible and can weaken trust among countries. Protecting against these threats needs strong teamwork between different groups, but the sea makes this extra difficult because of legal and responsibility issues.

Helsinki Convention

Main article: Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea Area

In 1974, the seven countries around the Baltic Sea signed a special agreement to protect the sea from pollution. This was the first time all the pollution sources around a whole sea were covered by one agreement. It started working on May 3, 1980.

Later, in 1992, a new agreement was signed by all the countries around the Baltic Sea and the European Community. This new agreement started working on January 17, 2000. It helps keep the whole Baltic Sea area clean, including the water, the sea bottom, and the land that drains into the sea. A group called the Helsinki Commission, also known as HELCOM, helps manage this agreement. The countries that agreed to follow it are Denmark, Estonia, the European Community, Finland, Germany, Latvia, Lithuania, Poland, Russia, and Sweden.

Coordination in the Baltic Sea region

The European Union plays a big role in helping countries around the Baltic Sea region work together safely. After the Baltic States joined in 2004, the Baltic Sea became part of the EU. The EU started several projects to help share information and keep the waters safe, like MARSUR in 2006 and SUCBAS in 2009.

The North Atlantic Treaty Organisation (NATO) also helps keep the area secure. After Finland joined in 2023 and Sweden in 2024, most countries around the Baltic Sea are now NATO members. NATO has started new projects to protect important underwater structures and increase its military presence in the region. One big yearly event is BALTOPS, a naval exercise held in the Baltic Sea.

The Nordic Defence Cooperation (NORDEFCO) is a group of Nordic countries — Denmark, Finland, Iceland, Norway, and Sweden — that work together on defense. It started in 2009 and aims to improve each country’s defense and work better with others.

The Council of the Baltic Sea States is a group of ten European countries and the EU that meets regularly to discuss ways to make the region safer and more prosperous. It was created in 1992 and focuses on building a strong regional identity.