Indonesia is the world's largest archipelagic state, with 17,508 officially listed islands, spread along the equator between the Indian and Pacific oceans in Southeast Asia and Oceania. Its land area of 1,904,569 km² (735,358 sq mi) makes it the 14th-largest country, and an official mid-2025 estimate put its population at 284.4 million, the fourth-largest in the world after India, China and the United States. The capital is Jakarta, on Java, although a new capital called Nusantara has been under construction in East Kalimantan, on Borneo, since 2022. Indonesian, a standardised form of Malay, is the official language, but more than 700 languages are spoken. Its most striking facts are geological and biological: roughly 400 volcanoes, about 130 of them active, and the Komodo dragon, the largest lizard alive.
No single description fits a country this varied, so the facts below travel across it region by region, from Sumatra in the west to Papua in the east.
17,508 Islands Across 5,120 Kilometres
The number of islands depends on who is counting. Law No. 9 of 1996 on Indonesia's maritime territory lists 17,508 islands, the same figure the CIA World Factbook uses. A survey by the national space institute LAPAN in 2002 found 18,307, while a geospatial survey from 2007 to 2010 counted 13,466; the gap comes mostly from whether sandbanks and reefs that appear only at low tide are included. As of 2023, the Geospatial Information Agency had named 17,024 islands, and one government estimate counts only about 922 as permanently inhabited.
The archipelago stretches 5,120 km from east to west and 1,760 km from north to south. Its five main islands are Sumatra, Java, Sulawesi, Borneo (Kalimantan) and New Guinea. Four islands are shared with neighbours: Borneo with Malaysia and Brunei, Sebatik with Malaysia, Timor with East Timor and New Guinea with Papua New Guinea. Busy shipping lanes such as the Malacca, Sunda and Lombok straits pass between the islands.
Even the name is about islands. It joins India with the Greek nēsos, "island", to mean "Indian islands". In 1850 the English ethnologist George Windsor Earl proposed the word Indunesians, James Richardson Logan adopted Indonesia as a geographical term in the same journal, and the German ethnographer Adolf Bastian popularised it in a five-volume work published from 1884 to 1894.
Sumatra and the Toba Supervolcano
Sumatra is the largest island lying wholly inside Indonesia and the sixth-largest island in the world, at about 482,287 km² including its offshore islands. Its northern half holds the most violent piece of geology in the country. Lake Toba, in North Sumatra, fills the caldera of a supervolcano: it is about 100 km long, 30 km wide and up to 505 m deep, sitting at roughly 900 m above sea level, and is both the largest lake in Indonesia and the largest volcanic lake in the world.
The eruption that created it, dated to about 74,000 years ago, rated 8 on the volcanic explosivity index and is the largest explosive eruption known on Earth in the last 25 million years. It temporarily changed the global climate. Toba Caldera became a UNESCO Global Geopark in July 2020.
Sumatra's west coast faces the Sunda megathrust, a 5,500 km fault where the Indo-Australian Plate pushes beneath the Sunda Plate. The earthquake on that fault on 26 December 2004 sent a tsunami that devastated Aceh and North Sumatra.
Krakatoa, Tambora and 130 Active Volcanoes
Indonesia lies where the Eurasian, Indo-Australian, Pacific and Philippine Sea plates meet, part of the Pacific Ring of Fire. The result is a chain of roughly 400 volcanoes running from Sumatra through Java, Bali and Flores and looping round to northern Sulawesi, of which about 130 are considered active.
Two of the deadliest eruptions in modern history happened here. In April 1815, Mount Tambora on Sumbawa erupted with a volcanic explosivity index of 7, the most powerful eruption in recorded human history, throwing 37 to 45 km³ of rock into the atmosphere. The ash cooled the planet and produced the "Year Without a Summer" of 1816, with harvest failures in many parts of the world.
On 27 August 1883, Krakatoa in the Sunda Strait between Java and Sumatra destroyed more than 70 percent of its own island. The third explosion that morning, at 10:02, is the loudest sound in recorded history: it was heard 3,110 km away in Perth and about 4,800 km away on Rodrigues, near Mauritius, and its pressure wave circled the globe more than three times. At least 36,417 deaths are attributed to the eruption and the tsunamis it raised. In 1927 a new island, Anak Krakatau, "Child of Krakatoa", rose from the caldera, and its partial collapse caused the Sunda Strait tsunami of 2018.
Java: 56 Percent of the People on 7 Percent of the Land
Java is the most populous island on Earth. The 2020 census recorded 270.2 million Indonesians, and about 56 percent of them live on Java, which makes up only about 7 percent of the country's land. At roughly 1,192 people per km² in 2025, the island is about as densely settled as Bangladesh. Volcanic ash helps explain it: the same eruptions that threaten the island also enrich its farmland.
Mount Merapi, about 28 km north of Yogyakarta, is the most active volcano in Indonesia and has erupted regularly since 1548; its hazard to nearby towns earned it a place among the world's Decade Volcanoes. Not far from its slopes, near Muntilan, stands Borobudur, a 9th-century Mahayana Buddhist temple of nine stacked platforms, decorated with 2,672 relief panels and originally 504 Buddha statues.
Java also anchors the country's languages. The Javanese are the largest ethnic group, about 40 percent of the population, and Javanese is the most widely spoken local language. Yet the national language is Indonesian, chosen from Malay, long the trading lingua franca of the archipelago, and the national motto, Bhinneka Tunggal Ika, means "unity in diversity".
Jakarta's Sinking Coast and the Move to Nusantara
Jakarta lies on Java's north-west coast at the mouth of the Ciliwung, facing Jakarta Bay. The city proper covers about 662 km², but its metropolitan area, Jabodetabek, has more than 40 million people, the most populous urban area in the world. Much of it stands on a low alluvial plain where thirteen rivers run to the sea, and heavy groundwater extraction and the weight of buildings have made the land subside, worsening floods in the north of the city.
Those problems helped push the government to build a new capital. Nusantara, an Old Javanese word meaning "archipelago", is rising in the East Kalimantan regencies of Penajam North Paser and Kutai Kartanegara, next to the port of Balikpapan. Construction began in 2022, the project was estimated at Rp 523 trillion (about US$35 billion), and it is planned in five phases running to 2045, the centenary of independence. On 17 August 2024, Indonesia held its Independence Day ceremony there for the first time. Jakarta remains the capital until the move is complete; our guide to Jakarta covers the city itself.
Komodo and the Wallace Line in Nusa Tenggara
East of Java the islands of Nusa Tenggara, the Lesser Sundas, string out towards Timor. Between Bali and Lombok, only about 35 km apart, runs the Wallace Line, drawn in 1859 by the British naturalist Alfred Russel Wallace. West of it the animals are Asian; east of it begins Wallacea, a transition zone between Asia and Australia whose islands were populated only by species able to cross the straits between them.
Further east lives the Komodo dragon (Varanus komodoensis), the largest living lizard, with males reaching 3 m and up to 150 kg. It is found only on Komodo, Rinca, Flores, Gili Dasami and Gili Motang. Komodo National Park was founded in 1980 to protect it, covers 1,733 km² including 603 km² of land, and became a UNESCO World Heritage Site in 1991.
Adults hunt Javan rusa deer and feral pigs and also eat a great deal of carrion; the young spend their early years in trees to avoid larger dragons. There is more on the park and its islands in our Komodo National Park guide.
Maluku, the Original Spice Islands
Around the Banda and Molucca seas lie the Maluku Islands, or Moluccas, some of the most geologically complex and active regions in the world: they sit at the meeting point of four geological plates and two continental blocks, with coral reefs scattered through some of the deepest seas on Earth. The two largest islands, Halmahera and Seram, are sparsely populated, while the most developed, Ambon and Ternate, are small. These were the Spice Islands, the only source of nutmeg, mace and cloves, and their spices drew European traders into the archipelago from the 16th century. The Dutch fought to monopolise it, conquering the nutmeg-producing Banda Islands by force in 1621. Maluku formed one province until 1999, when North Maluku, with its capital at Sofifi on Halmahera, was split off.
Papua: Puncak Jaya and Raja Ampat
Indonesia's eastern end is the western half of New Guinea, which shares a continental shelf with Australia and was joined to it by a land bridge, as the landmass of Sahul, when Ice Age sea levels were low. Its central spine holds the country's highest mountain, Puncak Jaya, at 4,884 m (16,024 ft) in the Sudirman Range. It is the highest peak on any island in the world, and it is often listed among the Seven Summits as the high point of Oceania.
Off the north-west tip of New Guinea, the Raja Ampat archipelago spreads over more than 1,500 small islands, cays and shoals around the main islands of Misool, Salawati, Batanta and Waigeo. It lies within the Coral Triangle, the area of Southeast Asian seas with the richest marine life on Earth. From Toba's crater to Papua's coral reefs, this is one country built by volcanoes and divided by seas. Its most notorious eruption is traced further in our complete guide to Krakatoa.