Thailand is in the middle of mainland Southeast Asia. It has a total size of which is the 50th largest in the world. The land border is long with Myanmar, Cambodia, Laos and Malaysia. The nation's axial position influenced many aspects of Thailand's society and culture. It controls the only land route from Asia to Malaysia and Singapore. It has an exclusive economic zone of . Physical geography A fertile floodplain and tropical monsoon climate, ideally suited to wet-rice (tham na) cultivation, attracted settlers to this central area in preference to the marginal uplands and the highlands of the northern region or the Khorat Plateau to the northeast. By the 11th century AD, a number of loosely connected rice-growing and trading states flourished in the upper Chao Phraya Valley. They broke free from domination of the Khmer Empire, but from the middle of the 14th century gradually came under the control of the Ayutthaya Kingdom at the southern extremity of the floodplain. Successive capitals, built at various points along the river, became centers of great Thai kingdoms based on rice cultivation and foreign commerce. Unlike the neighboring Khmer and Burmese, the Thai continued to look outward across the Gulf of Thailand and the Andaman Sea toward foreign ports of trade. When European colonisation of Southeast Asia brought a new phase in Southeast Asian commerce in the late-1800s, Thailand (known then as Siam) was able to maintain its independence as a buffer zone between British-controlled Burma to the west and French-dominated Indochina to the east, but losing over 50% of its territory in the process. Most of the areas lost contained a non-Thai population (Khmer, Lao or Shan). The Thai-speaking heartland remains intact. Boundaries Land boundaries Total: Border countries: Myanmar , Cambodia , Laos , Malaysia Coastline Total: Maritime claims Territorial sea: Exclusive economic zone: and Continental shelf: depth or to the depth of exploitation Topography and drainage The most conspicuous features of Thailand's terrain are high mountains, a central plain, and an upland plateau. Mountains cover much of northern Thailand and extend along the Myanmar border down through the Kra Isthmus and the Malay Peninsula. The central plain is a lowland area drained by the Chao Phraya River and its tributaries, the country's principal river system, which feeds into the delta at the head of the Bay of Bangkok. The Chao Phraya system drains about one-third of the nation's territory.