The Stockholm Archipelago () is the largest archipelago in Sweden, and the second-largest archipelago in the Baltic Sea (the largest being the Archipelago Sea across the Baltic in Finland). Part of the archipelago has been designated as a Ramsar site since 1989. Water The water has a pH value of 7.0 (neutral) or slightly lower, to be compared with approximately 8.0 for seawater. Together with humic substances this sometimes causes a bit brownish water colour, especially in the inner parts. The salinity varies between freshwater and brackish water with poor salinity. In the outer parts of the archipelago, the salinity reaches around 0.6–0.7 per cent by weight, to be compared with at least 1.5 for beginning to taste salty and around 3.0 or more for proper oceanic water. Sea ice is regularly formed in the inner parts every winter. Geography The archipelago extends from Stockholm roughly to the east. In a north–south direction, it mainly follows the coastline of the Södermanland and Uppland provinces, reaching roughly from Öja island, south of Nynäshamn, to Väddö, north of Norrtälje. It is separated from Åland by a stretch of water named South Kvarken. A separate group of islands lies further north, near the town of Öregrund. Between Arholma and Landsort there are approximately 24,000 islands and islets. Some of the better-known islands are Dalarö, Finnhamn, , Grinda, Husarö, Ingarö, Ljusterö, Möja, , Rödlöga, Tynningö, Utö, Svartsö and Värmdö. The biggest towns of the archipelago, apart from Stockholm, are Nynäshamn, Vaxholm and Norrtälje. The village of Ytterby, famous among chemists for naming no fewer than four chemical elements (erbium, terbium, ytterbium and yttrium), is situated on Resarö in the Stockholm archipelago. The shipping routes from the Baltic to Stockholm pass through the archipelago. There are three main entrances suitable for deep-draught craft, namely, those near Landsort, Sandhamn, and Söderarm. History The Stockholm archipelago is a joint valley landscape that has been shaped – and is still being shaped – by post-glacial rebound. It was not until the Viking Age that the archipelago began to assume its present-day contours. The islands rise by about three millimeters each year. In 1719 the archipelago had an estimated population of 2,900, consisting mostly of fishermen. Today the archipelago is a popular holiday destination with some 50,000 holiday cottages (owned mainly by Stockholmers). The Stockholm Archipelago Foundation, dedicated to the preservation of the nature and culture of the archipelago,