The Northern Islands (north of Saipan) form the Northern Islands Municipality. The three main islands of the Southern Islands form the municipalities of Saipan,Tinian, and Rota, with uninhabited Aguijan forming part of Tinian municipality. Because of volcanic threat, the northern islands have been largely evacuated. Human habitation is limited to Agrihan, Pagan, and Alamagan, but population varies due to various economic factors, including children's education. The Census of April 2000 registered just six people in all of the Northern Islands municipality (then residing on Alamagan), and the Northern Islands' mayor office is located in "exile" on Saipan. Saipan, Tinian, and Rota have the only ports and harbors, and are the only permanently populated islands.