Southeast Asia is a subregion of Asia, consisting of the countries that are geographically south of China and Japan, east of India, west of Papua New Guinea, and north of Australia. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. The region is the only part of Asia that lies partly within the Southern Hemisphere, although the majority of it is in the Northern Hemisphere. In contemporary definition, Southeast Asia consists of two geographic regions:
The region lies near the intersection of geological plates, with both heavy seismic and volcanic activities. The Sunda Plate is the main plate of the region, featuring almost all Southeast Asian countries except Myanmar, northern Thailand, northern Laos, northern Vietnam, and northern Luzon of the Philippines. The mountain ranges in Myanmar, Thailand, and peninsular Malaysia are part of the Alpide belt, while the islands of the Philippines are part of the Pacific Ring of Fire. Both seismic belts meet in Indonesia, causing the region to have relatively high occurrences of earthquakes and volcanic eruptions.
Southeast Asia covers about 4.5 million km2 (1.7 million mi2), which is 10.5% of Asia or 3% of earth's total land area. Its total population is more than 641 million, about 8.5% of the world's population. It is the third most populous geographical region in the world after South Asia and East Asia. The region is culturally and ethnically diverse, with hundreds of languages spoken by different ethnic groups. Ten countries in the region are members of ASEAN, a regional organization established for economic, political, military, educational and cultural integration amongst its members.