Mindanao, island, the second largest (afterLuzon) in thePhilippines, in the southern part of thearchipelago, surrounded by theBohol,Philippine,Celebes, andSulu seas. Irregularly shaped, it measures 293 miles (471 km) north to south and 324 miles (521 km) east to west. The island is marked by peninsulas and is heavily indented by the Davao and Moro gulfs in the south and byIligan Bay in the north. The long, semicircularZamboanga Peninsula (west) extends southwesterly toward theSulu Archipelago andBorneo, and the Cotabato andSurigao peninsulas extend south and north, respectively.

Rugged, faulted mountains andvolcanoes occur in many areas.Mount Apo, at 9,692 feet (2,954 metres), is an active volcano in the southern part of the central highlands; it is the highest peak in the Philippines. The island has narrow coastal plains, and broad, fertile basins and extensive swamps are formed by theMindanao andAgusan river systems.Lake Lanao (Lake Sultan Alonto), created by a lava dam, has an area of 134 square miles (347 square km). The island has a marsh-game refuge and bird sanctuary. The rarePhilippine eagle is found on Mindanao.

Mindanao is aMuslim outpost in the predominantlyRoman Catholic Philippines. Although Muslims are no longer a majority, Islamicculture is evident; there are many mosques, and distinctive brassware, including the kris, or dagger, is manufactured. Theautonomous region ofMuslim Mindanao—consisting of territory in western and southwestern Mindanao along with a number of nearby islands, includingTawi Tawi andJolo—was established in 1990.

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Mindanao also has the largest concentration of ethnic minorities in the Philippines. They include theMagindanao,Maranao, Ilanun, and Sangil; all are Muslim groups sometimes collectively called theMoro. Groups usually found in the uplands include the T’boli, Subanon, Bukidnon, Bagobo, Mandaya, and Manobo. Another important group is the Tiruray, whose religion is a mixture of Christian, Muslim, and local beliefs.

Because of its large expanses of undeveloped fertile land, Mindanao has been considered the country’s “pioneer frontier.” It did not experience substantial population increase until migration was promoted, particularly in the mid-20th century. The chief crops are corn (maize), rice, abaca, bananas, pineapples, mangoes, and coconuts. Cotton, ramie (a fibre plant), coffee, and cacao are also grown. The timber industry is important, and there are gold, nickel, iron, and coal deposits. Chartered cities includeZamboanga City,Cotabato City,Davao City,Cagayan de Oro, andButuan. Area 36,537 square miles (94,630 square km). Pop. (2015) 24,135,775.