Wikipedia
Söderarm is the name of a deactivated Swedish lighthouse station and its surrounding small archipelago in the west part of the sea of Åland. The light station is located on the island of Thorskär. This area, together with the tiny island of Tjärven north of Söderarm, marks the entrance for the shipping route leading to the ports of Kapellskär, Norrtälje, and also the north entrance to Stockholm. The area has been severely fortified by the Swedish military and coastal artillery, and many traces of military activity from the years during the second world war are still visible. On the island of Manskär, for example, a large military radar mast stood for many years. It was demolished in 2010. In 2002, the coastal artillery was formally liquidated at Söderarm, but still some islands are forbidden for the public to visit. The Swedish Maritime Administration has confirmed that many Russian mines dating from the first world war may still lie on the sea bottom east of the light station, making anchoring or diving there dangerous.