Indonesia’s 18,108 islands, of which about 6,000 are inhabited, are scattered around the equator, giving the country a tropical climate. The largest populated islands are Java, one of the most densely populated regions on Earth, where about half of the population lives, Sumatra, Borneo (shared with Malaysia and Brunei), New Guinea (shared with Papua New Guinea) and Sulawesi. The country borders Malaysia on the island of Borneo (Indonesian: Kalimantan), Papua New Guinea on the island of New Guinea and East Timor on the island of Timor. In addition to the capital city of Jakarta, principal Indonesian cities of high population include Surabaya, Bandung, Medan, Palembang, and Semarang.
Its location on the edges of tectonic plates, specifically the Pacific, Eurasian, and Australian, means Indonesia is frequently hit by earthquakes and the resulting tsunamis. Indonesia is also rich in volcanoes, the most famous being the now vanished Krakatau (Krakatoa), which was located between Sumatra and Java.
Flora and fauna differ markedly between Kalimantan, Bali, and western islands on the one hand and Sulawesi, Lombok, and islands further to the east on the other hand. This ecological boundary has been called the Wallace line after its discoverer. The line is often given as the boundary between Asia and Australia, as such making Indonesia a bi continental country.