which has a significant breeding population in the Archipelago Sea. Rare or endangered bird and mammal species found in the archipelago also include Caspian tern, greater scaup, grey seal and ringed seal. The islands are a haven for seabirds. Species include mute swan, black guillemot, great crested grebe and numerous species of gulls. Recently great cormorants have spread to the archipelago and their numbers are increasing. This not necessarily viewed as a good thing by nature lovers, since great cormorants live in dense colonies which will eventually poison surrounding plant life by their excrement. The greatest threat to the environment is eutrophication caused mainly by agriculture and fish farms. This is a particular threat to the Baltic Sea, since it is very shallow and thus has much less potential to dilute effluent from human activities. Eutrophication has been partly brought under control in Finland, but the effects are masked by the general decline in the condition of the Baltic Sea. Many areas of the archipelago are protected from human activity by their sheer inaccessibility. The southern part of the Archipelago Sea belong to the cooperation area of the Archipelago National Park and there are many small nature conservation areas, where landing is prohibited in spring and summer. History Prehistory The islands began rising from the water 10,000 years ago. At that stage the Danish Straits were closed and the current Baltic Sea was the fresh-water Lake Ancylus. The water around the islands turned from fresh to brackish around 7600 years ago as the saline water from the North Sea penetrated further to the Baltic from the recently opened straits. The oldest archaeological finds in Dragsfjärd date back to ca. 4000 BC and represent the Pit-Comb Ware culture. During that period the outer archipelago was formed by the highest points of the main islands of Houtskär, Korpo and Nagu. Due to post-glacial rebound the entire archipelago has risen approximately since, enlarging existing islands and creating many more. Swedish rule During the 12th and 13th centuries Sweden established its control of the Archipelago Sea. At the same time Christianity started to spread to Finland, starting from the archipelago and the adjacent coastal area. The islands occupy a strategic position, guarding the approaches to Stockholm, Turku and the entire Gulf of Bothnia. Therefore, they were fortified by the Swedish empire during the Middle Ages. A royal postal route used to go via the