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Indonesia lies between latitudes 11°S and 6°N, and longitudes 95°E and 141°E. It consists of 17,508 islands, about 6,000 of which are inhabited.^[86] These are scattered over both sides of the equator. The largest are Java, Sumatra, Borneo (shared with Brunei and Malaysia), New Guinea (shared with Papua New Guinea), and Sulawesi. Indonesia shares land borders with Malaysia on Borneo, Papua New Guinea on the island of New Guinea, and East Timor on the island of Timor. Indonesia shares maritime borders across narrow straits with Singapore, Malaysia, the Philippines, and Palau to the north, and with Australia to the south. The capital, Jakarta, is on Java and is the nation's largest city, followed by Surabaya, Bandung, Medan, and Semarang.^[87]
At 1,919,440 square kilometers (741,050 sq mi), Indonesia is the world's 16th-largest country in terms of land area.^[88] Its average population density is 134 people per square kilometer (347 per sq mi), 79th in the world,^[89] although Java, the world's most populous island,^[90] has a population density of 940 people per square kilometer (2,435 per sq mi). At 4,884 metres (16,024 ft), Puncak Jaya in Papua is Indonesia's highest peak, and Lake Toba in Sumatra its largest lake, with an area of 1,145 square kilometers (442 sq mi). The country's largest rivers are in Kalimantan, and include the Mahakam and Barito; such rivers are communication and transport links between the island's river settlements.^[91]

Mount Semeru and Mount Bromo in East Java. Indonesia's seismic and volcanic activity is among the world's highest.

Indonesia's location on the edges of the Pacific, Eurasian, and Australian tectonic plates makes it the site of numerous volcanoes and frequent earthquakes. Indonesia has at least 150 active volcanoes,^[92] including Krakatoa and Tambora, both famous for their devastating eruptions in the 19th century. The eruption of the Toba supervolcano, approximately 70,000 years ago, was one of the largest eruptions ever, and a global catastrophe. Recent disasters due to seismic activity include the 2004 tsunami that killed an estimated 167,736 in northern Sumatra,^[93] and the Yogyakarta earthquake in 2006. However, volcanic ash is a major contributor to the high agricultural fertility that has historically sustained the high population densities of Java and Bali.^[94]
Lying along the equator, Indonesia has a tropical climate, with two distinct monsoonal wet and dry seasons. Average annual rainfall in the lowlands varies from 1,780–3,175 millimeters (70–125 in), and up to 6,100 millimeters (240 in) in mountainous regions. Mountainous areas—particularly in the west coast of Sumatra, West Java, Kalimantan, Sulawesi, and Papua—receive the highest rainfall. Humidity is generally high, averaging about 80%. Temperatures vary little throughout the year; the average daily temperature range of Jakarta is 26–30 °C (79–86 °F).^[95]

Biota and environment

Main articles: Fauna of Indonesia, Flora of Indonesia, and Environment of Indonesia

The critically endangered Sumatran Orangutan, a great ape endemic to Indonesia.

Indonesia's size, tropical climate, and archipelagic geography, support the world's second highest level of biodiversity (after Brazil),^[96] and its flora and fauna is a mixture of Asian and Australasian species.^[97] The islands of the Sunda Shelf (Sumatra, Java, Borneo, and Bali) were once linked to the Asian mainland, and have a wealth of Asian fauna. Large species such as the tiger, rhinoceros, orangutan, elephant, and leopard, were once abundant as far east as Bali, but numbers and distribution have dwindled drastically. Forests cover approximately 60% of the country.^[98] In Sumatra and Kalimantan, these are predominantly of Asian species. However, the forests of the smaller, and more densely populated Java, have largely been removed for human habitation and agriculture. Sulawesi, Nusa Tenggara, and Maluku—having been long separated from the continental landmasses—have developed their own unique flora and fauna.^[99] Papua was part of the Australian landmass, and is home to a unique fauna and flora closely related to that of Australia, including over 600 bird species.^[100]
Indonesia is second only to Australia in terms of total endemic species, with 36% of its 1,531 species of bird and 39% of its 515 species of mammal being endemic.^[101] Indonesia's 80,000 kilometers (50,000 mi) of coastline are surrounded by tropical seas that contribute to the country's high level of biodiversity. Indonesia has a range of sea and coastal ecosystems, including beaches, sand dunes, estuaries, mangroves, coral reefs, sea grass beds, coastal mudflats, tidal flats, algal beds, and small island ecosystems.^[8] Indonesia is one of Coral Triangle countries with the world's greatest diversity of coral reef fish with more than 1,650 species in eastern Indonesia only.^[102] The British naturalist, Alfred Wallace, described a dividing line between the distribution and peace of Indonesia's Asian and Australasian species.^[103] Known as the Wallace Line, it runs roughly north-south along the edge of the Sunda Shelf, between Kalimantan and Sulawesi, and along the deep Lombok Strait, between Lombok and Bali. West of the line the flora and fauna are more Asian; moving east from Lombok, they are increasingly Australian. In his 1869 book, The Malay Archipelago, Wallace described numerous species unique to the area.^[104] The region of islands between his line and New Guinea is now termed Wallacea.^[103]
Indonesia's high population and rapid industrialization present serious environmental issues, which are often given a lower priority due to high poverty levels and weak, under-resourced governance.^[105] Issues include large-scale deforestation (much of it illegal) and related wildfires causing heavy smog over parts of western Indonesia, Malaysia and Singapore; over-exploitation of marine resources; and environmental problems associated with rapid urbanization and economic development, including air pollution, traffic congestion, garbage management, and reliable water and waste water services.^[105] Deforestation and the destruction of peatlands make Indonesia the world's third largest emitter of greenhouse gases.^[106] Habitat destruction threatens the survival of indigenous and endemic species, including 140 species of mammals identified by the World Conservation Union (IUCN) as threatened, and 15 identified as critically endangered, including Bali Starling,^[107] Sumatran Orangutan,^[108] and Javan Rhinoceros.^[107] Much of Indonesia's deforestation is caused by forest clearing for the palm oil Industry, which has cleared 18 million hectares of forest for palm oil expansion. Palm oil expansion requires land reallocation as well as changes to the local and natural ecosystems. Palm oil expansion can generate wealth for local communities if done right. If down wrong it can degrade ecosystems and cause social conflicts.^[109]