The Free Aceh Movement fought against Indonesian government forces in the Aceh Insurgency from 1976 to 2005. Security crackdowns in 2001 and 2002 resulted in several thousand civilian deaths. The island was heavily impacted by both the 1883 Krakatoa eruption and the 2004 Boxing Day Tsunami. Demographics Sumatra is not particularly densely populated, with 125 people per km2 – about 59,185,500 people in total (according to official estimates as at mid 2021). Because of its size, it is nonetheless the fifth most populous island in the world. Ethnic groups The largest indigenous ethnic groups in Sumatra are Malays, Minangkabaus, Bataks, Acehnese, and Lampungs. Other major non-indigenous ethnic groups are Javanese, Sundanese, and Chinese. Below are 11 largest ethnic groups in Sumatra based on the 2010 census (including Riau Islands, Bangka Belitung, Nias, Mentawai, Simeulue and islands around it) Languages There are over 52 languages spoken, all of which (except Chinese and Tamil) belong to the Malayo-Polynesian branch of the Austronesian language family. Within Malayo-Polynesian, they are divided into several sub-branches: Chamic (which are represented by Acehnese in which its closest relatives are languages spoken by Ethnic Chams in Cambodia and Vietnam), Malayic (Malay, Minangkabau and other closely related languages), Northwest Sumatra–Barrier Islands (Batak languages, Gayo and others), Lampungic (includes Proper Lampung and Komering) and Bornean (represented by Rejang in which its closest linguistic relatives are Bukar Sadong and Land Dayak spoken in West Kalimantan and Sarawak (Malaysia)). Northwest Sumatra–Barrier Islands and Lampungic branches are endemic to the island. Like all parts of Indonesia, Indonesian (which was based on Riau Malay) is the official language and the main lingua franca. Although Sumatra has its own local lingua franca, variants of Malay like Medan Malay and Palembang Malay are popular in North and South Sumatra, especially in urban areas. Minangkabau (Padang dialect) is popular in West Sumatra, some parts of North Sumatra, Bengkulu, Jambi and Riau (especially in Pekanbaru and areas bordered with West Sumatra) while Acehnese is also used as an inter-ethnic means of communication in some parts of Aceh province. Religion The majority of people in Sumatra are Muslims (87.1%), while 10.7% are Christians, and less than 2% are Buddhists and Hindus. Administration Sumatra is one of seven geographical regions of Indonesia which includes its adjacent smaller islands. Sumatra was one of the eight original provinces of Indonesia between 1945 and 1948. Including adjacent archipelagoes normally included with Sumatra (such