Abuja is the capital city of Nigeria. It is located in the centre of Nigeria, within the Federal Capital Territory. Abuja is a planned city, and was built mainly in the 1980s. It officially became Nigeria's capital on 12 December 1991, replacing Lagos, which is still the country's most populous city. At the 2006 census, the city of Abuja had a population of 776,298, making it one of the ten most populous cities in Nigeria. Abuja has witnessed a huge influx of people into the city which has led to the emergence of satellite towns such as Karu Urban Area, Suleja, Gwagwalada, Lugbe, Kuje and smaller settlements to which the planned city is sprawling towards. The unofficial metropolitan area of Abuja has a population of well over three million, which makes it the fourth largest urban area in Nigeria, only suppassed by Lagos, Kano and Ibadan.Abuja's geography is defined by Aso Rock, a 400-metre monolith left by water erosion. The Presidential Complex, National Assembly, Supreme Court and much of the town extend to the south of the rock.Other sights include the Nigerian National Mosque and the Nigerian National Christian Centre. The city is served by the Nnamdi Azikiwe International Airport, while Zuma Rock lies nearby. Abuja is known for being the best purpose-built city in Africa as well as being one of the wealthiest and most expensive; however, the population on the semi-developed edges of the city are living in rural areas such as Mararaba and Karu, Nasarawa State.