Açores, islands (1991 pop. 241,592), 905 sq mi (2,344 sq km), in the Atlantic Ocean, c.900 mi (1,448 km) W of mainland Portugal. Administratively an autonomous region of Portugal, they are divided into three districts named after their capitals: Ponta Delgada on São Miguel Angra do Herosmo on Terceira Horta on Faial The nine main islands are São Miguel (the largest) and Santa Maria in the southeast; Terceira, Pico, Faial, São Jorge, and Graciosa in the center; and Flores and Corvo in the northwest. Ponta Delgada is the largest city. The fertile soil yields many crops and supports vineyards. The islands are also a resort area, although there is volcanic activity. The United States maintains a NATO air base in the islands. The Azores may have been known to the ancients and were included on a map in 1351. Portuguese sailors reached them in 1427 or 1431, but colonization did not begin until 1445 under Diego de Sevilha or Gonalo Velho Cabral (who may have been there in 1431). During the colonial period, the Azores were a stopover point for treasure fleets returning from the New World, and many ships were lost there due to hurricanes or pirates. The islands were used as a place of exile and were also the site of naval battles between the English and the Spanish. In the 19th century, they were used by supporters of Maria II against Dom Miguel. In the 20th century, there has been a large outmigration to the United States.