Geologically, the lake was formed after the Quaternary glaciation about 10,000 years ago; when the ice melted, the entire width of Sweden was covered in water, creating a strait between Kattegat and the Gulf of Bothnia.
Xeoloxicamente o lago formouse despois da derradeira glaciación, hai uns 10.000 anos; cando o xeo se fundiu toda a anchura de Suecia cubríuse de auga, creando un estreito entre Kattegat e o golfo de Botnia.