Archipelago Sea (Finnish Saaristomeri, Swedish Skärgårdshavet) is a part of the Baltic Sea between the Gulf of Bothnia, the Gulf of Finland and the Sea of Åland, within Finnish territorial waters. By some definitions it contains the largest archipelago (island group) in the world by the number of islands, although many of the islands are very small and tightly clustered.
Merenkurkku / Kvarken archipelago, Finland. Known for its unique geological situation. The seabed is one of world's fastest to rise up from the sea, 1 cm/year, after last ice age. It makes new islands outside Finland all the time. The fishermen have difficulties to navigate between those and a big part of the western coastline of Finland is moving far out to the sea. The mainland is increasing. Listed on the UNESCO Heritage List in both Sweden and Finland.