Macau, also spelled Macao ( (); 澳門, Cantonese: [ōu.mǔːn]; official Portuguese: [mɐˈkaw] Macau), and officially the Macao Special Administrative Region of the People’s Republic of China, is a city in the western Pearl River Delta by the South China Sea. It is a special administrative region of China and maintains separate governing and economic systems from those of mainland China. With a population of 696,100 and an area of 32.9 km2 (12.7 sq mi), it is the most densely populated region in the world. Macau was formerly a colony of the Portuguese Empire, after Ming China leased the territory as a trading post in 1557. Portugal paid an annual rent and administered the territory under Chinese sovereignty until 1887 when it gained perpetual colonial rights in the Sino-Portuguese Treaty of Peking. The colony remained under Portuguese rule until 1999, when it was transferred to China. Originally a sparsely populated collection of coastal islands, the territory has become a major resort city and the top destination for gambling tourism. Its gambling industry is seven times larger than that of Las Vegas. Although the city has one of the highest per capita incomes in the world, it has severe income inequality. Its GDP per capita by purchasing power parity is one of the highest in the world and higher than any country in the world in 2014 according to the World Bank. Macau has a very high Human Development Index, although it is only calculated by the Macau government instead of the United Nations. Macau has the fourth-highest life expectancy in the world. The territory is highly urbanised and most development is built on reclaimed land; two-thirds of the total land area is reclaimed from the sea.