is smaller than for example than in Kvarken (Finnish: Merenkurkku) further north. The sea area is shallow, with a mean depth of 23 m. Most of the channels are not navigable for large ships. There are three crater-like formations in the archipelago. One of them, Lumparn in Åland, is a genuine impact crater. The two other formations are intrusions. The more prominent of these is the Åva Intrusion in the municipality of Brändö, which is visible in satellite photos and high-resolution maps. The other similar formation is in Fjälskär, between the main islands of Houtskär and Iniö. Administration The islands are divided between the region of Southwest Finland and the autonomous region of Åland. The border between the regions runs roughly along Skiftet (Kihti in Finnish), a relatively open sea area. Together with the islands near the coast of Sweden the area forms a Euroregion. The main ports in the area are Turku on the continent, and Mariehamn on Åland. Åland The Åland region is autonomous and demilitarized. It has its own regional parliament and has Swedish as its sole official language. The regional parliament has power over wide-ranging matters, including health services, education, environment, and postal services. Monetary and foreign policy are handled by the Parliament of Finland. The president of Finland has, in theory, right to veto the laws passed by the Åland regional parliament. Defence The eastern part of the archipelago is defended by the Archipelago Sea Naval Command, which has its main base in Turku. The defence is based largely on naval mines and coastal artillery. Both are effective in the archipelago, where the dense clusters of islands severely limit the manoeuvrability of invading vessels. The autonomous region of Åland is demilitarised. The Finnish Defence Forces are not allowed to enter the territory of Åland in peacetime (in times of war the FDF has the duty of defending Åland due to international treaties starting from the Åland crisis which resulted in Finland securing possession of the islands while becoming obliged to protect the islands' neutrality), and its residents are exempt from military service, although they can volunteer to serve in the army. Municipalities The archipelago is divided into 30 municipalities, grouped in the autonomous region of Åland and in the historical provinces of Varsinais-Suomi and Uusimaa. The municipalities in Åland tend to be quite small, with the municipality of Sottunga having only approximately 100 residents.Island municipalities