The heart of the country is a saucer-shaped, gently rolling alluvial plain drained by the Mekong River and enclosed by mountain ranges; the Dangrek Mts. form the frontier with Thailand in the northwest and the Cardamom Mts. and the Elephant Range are in the southwest. About half the land is tropical forest. In general, Cambodia has a tropical monsoon climate, with the wet southwest monsoon occurring between November and April and the dry northeast monsoon the remainder of the year. During the rainy season the Mekong swells and backs into the Tônlé Sap (Great Lake), increasing the size of the lake almost threefold. The seasonal rise of the Mekong floods almost 400,000 acres (162,000 hectares) around the lake, leaving rich silt when the waters recede.
One of the few underpopulated countries of Southeast Asia, Cambodia is inhabited by Cambodians (or Khmers), who comprise about 90% of the population. About 5% of the people are Vietnamese and 1% are Chinese; other ethnic groups include the Cham-Malays and the hill tribespeople. Theravada Buddhism is the state religion, but religious freedom is constitutionally guaranteed. About 95% of the people are Buddhists; the Cham-Malays are Muslims. Khmer is the official language, but French and English are widely used.