Short answers, pulled from the story.
The Baltic Sea is the world's largest brackish basin stretching from 53°N to 66°N latitude. Its low salinity prevents widespread species like the common starfish or shore crab from surviving in most parts of the sea.
Norse merchants built a vast trade empire surrounding the entire coastline of this inland sea during the Viking Age. The Hanseatic League emerged as the dominant economic force between the 13th and 16th centuries to control commerce flowing through the region.
The sinking of the Wilhelm Gustloff remains the worst maritime disaster in history killing roughly 9,000 people during World War II. Only 350 survived of the 4,500 prisoners who had been aboard the Cap Arcona when it was attacked on the 3rd of May 1945.
Tens of thousands of tons of German ammunition were dumped after the war at the behest of Allied powers to prevent future conflicts. Deteriorating bottles leak mustard gas and other substances slowly poisoning substantial parts of the water column.
In September 2022 both Nord Stream I and Nord Stream II were damaged by explosives close to Bornholm in Denmark. Another incident involved damage to undersea communication cables in late 2024 suspected to be caused by the oil tanker Eagle S linked to a Russian shadow fleet.