Short answers, pulled from the story.
The Baltic Sea is classified as the world's largest brackish water basin because its water is a mix of fresh and salt water, with surface salinity running between 0.3 and 0.9%, far below the ocean average of 3.5%. More than 250 rivers drain into it, contributing about 660 cubic kilometers of fresh water per year, while limited water exchange through the narrow Danish Straits prevents full saltwater replacement.
Adam of Bremen, an eleventh-century German chronicler, was the first to record the name Mare Balticum. He explained it by comparing the sea to a belt, drawing on the Latin word balteus, noting that the sea stretched through the land in a long belt-like shape.
The sinking of the Wilhelm Gustloff in 1945 is the worst maritime disaster in recorded history, killing roughly 9,000 people. The ship was carrying retreating soldiers and refugees when it was torpedoed during the final months of World War II.
Between 2022 and 2024, a series of incidents damaged critical Baltic Sea infrastructure. In September 2022, underwater explosions destroyed both Nord Stream 1 and Nord Stream 2 near Bornholm, Denmark. In October 2023, the anchor of the container vessel Newnew Polar Bear damaged the Balticconnector gas pipeline. In December 2024, the Estlink 2 cable was ruptured, with suspicion falling on the tanker Eagle S, believed to be part of a Russian shadow fleet.
On average, the Baltic Sea is ice-covered over about 45% of its surface at the annual maximum, typically in February or March. Freezing begins in the northern Gulf of Bothnia in mid-November and progresses south. Since 1720, the entire Baltic has frozen over just 20 times, most recently in early 1987, when ice covered 400,000 square kilometers.
Only two or three species are known to be endemic to the Baltic Sea: the brown alga Fucus radicans and the flounder Platichthys solemdali. Both were recognized as distinct species only recently, in 2005 and 2018 respectively, having previously been misidentified as more widespread relatives. A rare mussel, the Copenhagen cockle, is sometimes also considered endemic.