core of Rio, lies on the plains of the western shore of Guanabara Bay. The greater portion of the city, commonly referred to as the North Zone (), extends to the northwest on plains composed of marine and continental sediments and on hills and several rocky mountains. The South Zone (Zona Sul) of the city, reaching the beaches fringing the open sea, is cut off from the center and from the North Zone by coastal mountains. These mountains and hills are offshoots of the Serra do Mar to the northwest, the ancient gneiss-granite mountain chain that forms the southern slopes of the Brazilian Highlands. The large West Zone (Zona Oeste), long cut off by the mountainous terrain, had been made more easily accessible to those on the South Zone by new roads and tunnels by the end of the 20th century. The population of the city of Rio de Janeiro, occupying an area of , is about 6,000,000. The population of the greater metropolitan area is estimated at 11–13.5 million. Residents of the city are known as cariocas. The official song of Rio is "Cidade Maravilhosa", by composer André Filho. Parks The city has parks and ecological reserves such as the Tijuca National Park, the world's first urban forest and UNESCO Environmental Heritage and Biosphere Reserve; Pedra Branca State Park, which houses the highest point of Rio de Janeiro, the peak of Pedra Branca; the Quinta da Boa Vista complex; the Botanical Garden; Rio's Zoo; Parque Lage; and the Passeio Público, the first public park in the Americas. In addition the Flamengo Park is the largest landfill in the city, extending from the center to the south zone, and containing museums and monuments, in addition to much vegetation. Since 1961, the Tijuca National Park (Parque Nacional da Tijuca), the largest city-surrounded urban forest and the second largest urban forest in the world, has been a National Park. The largest urban forest in the world is the Floresta da Pedra Branca (White Rock Forest), which is located in the West Zone of Rio de Janeiro. Environment Due to the high concentration of industries in the metropolitan region, the city has faced serious problems of environmental pollution. The Guanabara Bay has lost mangrove areas and suffers from residues from domestic and industrial sewage, oils and heavy metals. Although its waters renew when they reach the sea, the bay is the final receiver