Brazil forms its eastern border; its other neighbors are Peru and Chile on the west and Argentina and Paraguay on the south. The western part, enclosed by two chains of the Andes, is a great plateau—the Altiplano, with an average altitude of 12,000 ft (3,658 m). Almost half the population lives on the plateau, which contains Oruro, Potosí, and La Paz. At an altitude of 11,910 ft (3,630 m), La Paz is the highest administrative capital city in the world. The Oriente, a lowland region ranging from rain forests to grasslands, comprises the northern and eastern two-thirds of the country. Lake Titicaca, at an altitude of 12,507 ft (3,812 m), is the highest commercially navigable body of water in the world.