El Gulf of Bothnia is a gulf located between western Finland and eastern Sweden. Its surface area is 116.300 km², it is 725 km long, between 80 and 240 km wide and an average depth of 60 m, its maximum being 295. It is the northernmost arm of the Baltic Sea. Its waters are shallow, quite cold, the northernmost part remaining frozen for 5 months a year, and low salinity, and various species of freshwater fish can even live in its waters.
Botnia is a latinization of the Old Norse language expression botn, meaning "low." The name botn was applied to the gulf as Helsingjabotn in the old Norse language, as opposed to Hälsingland, which was the name given to the coastal area west of the gulf. Subsequently, botten was applied to the regions Västerbotten in the western part and Österbotten in the eastern part ("East Bottom" and "West Bottom"). The Finnish name Österbotten, Pohjanmaa, or "Pohja" -land, provides a clue to its meaning in both languages: pohja means "low" and "North" at the same time.
The Gulf of Bothnia, together with the Baltic Sea, is part of those that prehistorically, until the Pleistocene, formed the wide plain of the river basin of the Eridanos River. This river originated in the Lapland region, flowed through what is now the Gulf of Bothnia, and emptied into the North Sea forming a delta of immense proportions.
From the Pleistocene there were several episodes of glacierism through which the area sank below sea level due to the weight of the ice. This happened about 700.000 years ago. From then on, the characteristics that determined what the current gulf is were made up of the weight of the ice sheet that sank the region and the subsequent isostatic adjustment.