Bolivia is the world's 28th-largest country, and the fifth largest country in South America. One-third of the country is the Andean mountain range. The geography of the country exhibits a great variety of terrain and climates. Bolivia has a high level of biodiversity, considered one of the greatest in the world, as well as several ecoregions with ecological sub-units such as the Altiplano, tropical rainforests (including Amazon rainforest), dry valleys, and the Chiquitania, which is a tropical savanna. These areas feature enormous variations in altitude, from an elevation of 6,542 metres (21,463 ft) above sea level in Nevado Sajama to nearly 70 metres (230 ft) along the Paraguay River. The country's population, estimated at 11 million, is multiethnic, including Amerindians, Mestizos, Europeans, Asians and Africans. It is the poorest country in South America with a poverty level of 53 percent and around 4 Million people are under 18.