Short answers, pulled from the story.
Mainland Southeast Asia comprises Cambodia, Laos, Myanmar, Peninsular Malaysia, Singapore, Thailand, and Vietnam. All seven countries are members of ASEAN. Together they cover over 2 million square kilometres and hold a population of around 277 million people as of 2025.
The term Indochina was coined in the early nineteenth century by Danish-French geographer Conrad Malte-Brun and Scottish linguist John Leyden, who saw the region as a cultural blend of Indian and Chinese influence. France later adopted it as the official name of its colony covering present-day Cambodia, Laos, and Vietnam. After the Vietnam War, the term gradually gave way to the more neutral Mainland Southeast Asia.
The Mekong is the longest river in Southeast Asia at 4,900 kilometres. It originates in the Tibetan Plateau and flows through Myanmar, Laos, Thailand, Cambodia, and Vietnam. Its basin drains 795,000 square kilometres and the river carries the world's tenth largest mean annual discharge of 475 cubic kilometres.
The mainland is home to three major language families: Austroasiatic (spoken by the Khmer and Viet), Tai-Kadai (spoken by the Thai and Lao), and Sino-Tibetan (which includes Burmese). Malay, the dominant language of Peninsular Malaysia and Singapore, belongs to the Austronesian family. Despite these distinct origins, the languages of the mainland have converged over centuries and share typological similarities.
Hkakabo Razi in Myanmar is the highest point in Mainland Southeast Asia, reaching 5,881 metres. The lowest point in the region is the Boh Yai mine in Kanchanaburi Province, Thailand.
Tonle Sap in Cambodia is the largest freshwater lake in Southeast Asia, with a maximum surface area of 16,000 square kilometres. It is located in the interior of Cambodia and is a central feature of the country's geography.