Yunnan is a province of the People's Republic of China, located in the far southwest of the country spanning approximately 394,000 square kilometers (152,000 sq mi) and with a population of 45.7 million (2009). The capital of the province is Kunming. The province borders Myanmar, Laos, and Vietnam. Yunnan is situated in a mountainous area, with high elevations in the northwest and low elevations in the southeast. Most of the population lives in the eastern part of the province. In the west, the relative height from mountain peaks to river valleys can be as much as 3,000 meters (9,800 ft). Yunnan is rich in natural resources and has the largest diversity of plant life in China. Of the approximately 30,000 species of higher plants in China, Yunnan has perhaps 17,000 or more. Yunnan's reserves of aluminium, lead, zinc and tin are the largest in China, and there are also major reserves of copper and nickel.Yunnan became part of the Han Dynasty (206 BC-220 AD) during 2nd century BC. It became the seat of a Tibeto-Burman speaking kingdom known as the Kingdom of Nanzhao in the 8th century AD. Nanzhao was multi-ethnic, but the elite most likely spoke a northern dialect of Yi, which became established as the prestige dialect. The Mongols conquered the region in the 13th century, with local control exercised by warlords until the 1930s. As with other parts of China's southwest, Japanese occupation in the north during World War II forced a migration of majority Han people into the region. Ethnic minorities in Yunnan account for about 34 percent of its total population. Major ethnic groups include Yi, Bai, Hani, Zhuang, Dai and Miao.