(mĭndӘnä'ō, –nou'), island (1990 pop. 13,535,738), 36,537 sq mi (94,631 sq km), second largest of the Philippine islands, NE of Borneo. About one fifth of the island's population is Muslim (see Moros). The terrain is generally mountainous and heavily forested, rising to 9,690 ft (2,954 m) at Mt. Apo, an active volcano and the highest point in the Philippines. The island is indented by several deep bays and has a large western peninsula, the Zamboanga or Sibuguey Peninsula. Its main rivers are the Mindanao (known as the Pulangi in its upper course), c.200 mi (320 km) long and navigable by small steamers for c.40 mi (60 km); and the Agusan, c.240 mi (390 km) long. The largest lake is Lake Lanao, for centuries the habitat of Muslim Moros. Off the northeast coast in the Philippine Sea is the Mindanao Trench (c.35,000 ft/10,670 m deep), one of the greatest known ocean depths.
Mindanao lies below the typhoon belt, and its climate is more favorable than that of Luzon to the north, but it has been been hit by tropical storms and typhoons. Bananas, pineapples, coconuts, mangoes, and other fruits are grown, as well as rice, corn, and coffee. Fish, especially tuna, and other marine products are harvested. Zamboanga and Davao are the principal cities; Davao is the most important port. There has been considerable industrial growth on the island since the 1960s. The extensive development of the water resources of the Lake Lanao–Agus River basin, including the harnessing of Maria Cristina Falls, has resulted in the establishment of heavy industrial plants, especially in the Iligan area. Mineral resources include gold, nickel, zinc and manganese.
In the middle of the 14th cent. Islam spread from Malaya and Borneo to the Sulu Archipelago, and from there to Mindanao. The arrival of the Spanish in the late 16th cent. united the various Muslim groups in a war against the conquerors that lasted some 300 years. The Moros likewise resisted American domination; fighting between U.S. garrisons and Muslim groups occurred early in the 20th cent.
Although many of the Philippine Islands suffered extensive damage in World War II, Mindanao emerged relatively unscathed. As the chief frontier left in the difficult reconstruction years, it was the object of government colonization projects. During the 1960s it experienced a phenomenal population increase and very rapid development. These changes brought serious problems. The native Moros, finding themselves outnumbered and in many cases pushed off their lands, retaliated with terrorist activities. When the Philippine army attempted to restore order, fierce fighting often resulted. In 1969 and the early 1970s several thousand people were killed and hundreds of villages were burned.
In 1971 anthropologists reported the discovery of the Tasaday, whom they portrayed as a Stone Age people inhabiting caves in Mindanao's rain forest and threatened by the encroachment of lumbering, mining, and ranching interests. By the mid-1980s, when evidence had emerged indicating that the Tasaday were perhaps a division of a neighboring, comparatively sophisticated people, there arose a suspicion that the Tasaday phenomenon was a hoax, possibly instigated by the Marcos government.
In 1976 the Philippine government pledged to grant autonomy to several provinces in Mindanao. It was not until 1990, following a plebiscite boycotted by many Muslims and dominated by Christian majorities in a number of provinces, that the Autonomous Region in Muslim Mindanao (now consisting of the Mindanao provinces of Maguindanao and Lanao del Sur and the Tawi-Tawi, Sulu, and Basilan provinces in the Sulu Archipelago) was granted partial autonomy. Muslim discontent with partial rule persisted, and unrest and violence continued through 1990s. A 1996 agreement led to peace with one group of rebels and the expansion of the autonomous region in 2001. Negotiations continued with another Moro group, but they and fundamentalist Islamic guerrillas have continued fighting and terror attacks.
An agreement reached (Nov., 2007) in principle with the second Moro group collapsed after it was challenged in court (and subsequently declared unconstitutional). Significant fighting broke out beginning in Aug., 2008, between government forces and some Moro rebels. Government military operations continued until mid-2009. Peace talks resumed late in 2009, but subsequently there were occasional outbreaks of fighting. In 2012 the government and the second Moro group signed a framework peace agreement that would create a new autonomous region superseding the current one, but that accord prompted a splinter group of the rebels who had signed the 1996 agreement to launch attacks in Sept., 2013. A 2014 peace agreement with the second group called for replacing the autonomous region by 2016 with a new one, to be called Bangsamoro, with somewhat enlarged territory and increased autonomy, but legislation creating Bangsamoro was stalled into 2016. Five months of devastating fighting erupted in Marawi between the military and hundreds of Islamic militants after a military raid in May, 2017, to capture the militants' leader failed.