Gulf of Bothnia
The Gulf of Bothnia is the northernmost extension of the Baltic Sea.
Positioned in Northern Europe, it's bordered by Sweden (a part of the Scandinavian Peninsula) and Finland.
Finland's Aland Islands (literally thousands of them), bordered by the Archipelago Sea and the Sea of Aland, stand at the southern edge of the Gulf of Bothnia.
Estimating the size of the Gulf of Bothnia is based on where its waters actually end, so all known measurements are estimates, at best.
Wikipedia references a surface area of 117,000 sq km, while Baltic Resources uses 116,165 sq km. Average depth runs near 60 meters (200 ft), while the maximum depth is measured at 295 meters (965 ft).
Numerous rivers drain into the gulf from both Finland and Sweden. Most of the northern areas freeze solid in winter, as its waters have less salinity than the southern reaches of the Baltic Sea.