Indonesia is an archipelagic country extending about 5,120 kilometres (3,181 mi) from east to west and 1,760 kilometres (1,094 mi) from north to south. According to a geospatial survey conducted between 2007 and 2010 by National Coordinating Agency for Survey and Mapping (Bakosurtanal), Indonesia has 13,466 islands. While earlier survey conducted in 2002 by National Institute of Aeronautics and Space (LAPAN) stated Indonesia has 18,307 islands. According to the CIA World Factbook, there are 17,508 islands. The discrepancy between the surveys is likely caused by the earlier different survey method including tidal islands, sandy cays and rocky reefs that surface during low tide and submerge during high tide. There are 8,844 named islands according to estimates made by the government of Indonesia, with 922 of those are permanent. It comprises five main islands: Sumatra, Java, Borneo (known as Kalimantan in Indonesia), Sulawesi, and New Guinea; two major island groups (Nusa Tenggara and the Maluku Islands) and sixty smaller island groups. Four of the islands are shared with other countries: Borneo is shared with Malaysia and Brunei; Sebatik, located off the northeastern coast of Kalimantan, shared with Malaysia; Timor is shared with East Timor; and New Guinea is shared with Papua New Guinea.
The topography of Indonesia’s islands varies, but it consists mainly of coastal lowlands. Some of Indonesia’s larger islands (Sumatra and Java for example) have large interior mountains. Because the 13,677 islands that makeup Indonesia are located on the two continental shelves, many of these mountains are volcanic, and there are several crater lakes on the islands. Java alone has 50 active volcanoes.
Because of its location, natural disasters—especially earthquakes—are common in Indonesia. On December 26, 2004, a 9.1 to 9.3 magnitude earthquake struck in the Indian Ocean, which triggered a large tsunami that devastated many Indonesian islands.
Indonesia’s climate is tropical with hot and humid weather in lower elevations. In the highlands of Indonesia’s islands, temperatures are more moderate. Indonesia also has a wet season that lasts from December to March.