The Gulf of Bothnia is the least known part of the Baltic Sea. In winter, the Baltic Sea is frozen for 300 kilometres. When Lapland's coast is still icy, millions of fish are drawn to the shore in the Kvarken archipelago 300 kilometres further south. Their destination is shallow beach lakes. Because they can only spawn where the water is really sweet. The Kvarken archipelago is also home to one of Europe's rarest animals, the sliding squirrel. Opposite on the Swedish side is the 'Höga Kusten', the "high coast". Here, red granite rises more than 300 metres out of the Baltic Sea. At the summer solstice, salmon migrate up the great Finnish rivers to their spawning grounds...