Short answers, pulled from the story.
The Baltic Sea is the world's largest brackish water basin where fresh river water meets salt ocean water in a slow, dangerous dance. It functions more like an inland sea with limited exchange with the open ocean, creating a fragile ecosystem where the water is too salty for some freshwater species but too fresh for many marine creatures.
The Viking Age began in the early Middle Ages when Scandinavians referred to the sea as Austmarr, or Eastern Sea, while the Vikings correctly identified it as an inlet known as Gandvik. This period saw the sea become the stage for a vast trade empire built by Norse merchants who navigated its waters to connect the North with the East.
The sinking of the Wilhelm Gustloff remains the worst maritime disaster in history, killing roughly 9,000 people. The burning of the Cap Arcona occurred shortly after the attacks on the 3rd of May 1945, seeing only 350 survivors of the 4,500 prisoners aboard.
The Soviet Union, the United Kingdom, and the United States disposed of chemical weapons into the Baltic Sea after the end of World War II. The most recent report from the Helsinki Commission notes that four small-scale catches of chemical munitions representing approximately 100 tons of material were reported in 2005.
The accession of Sweden and Finland to NATO occurred in 2023 and 2024, making the Baltic Sea almost entirely surrounded by the alliance's members. Russia designated the regional sea as a critical part of its naval doctrine published in 2022, leading some commentators to label the sea a NATO lake.
A massive algal bloom covering 100,000 square kilometers was revealed in July 2010. Eutrophication is caused by fertilizer runoff from surrounding agricultural land, which leads to massive algal blooms and threatens to turn the sea from a carbon sink to a source of carbon dioxide and methane.