Abuja (/əˈbuːdʒə/)[4] is the capital and sixth most populous city of Nigeria.[5] Situated in the middle of the country within the Federal Capital Territory (FCT), it is a planned city built mainly in the 1980s based on a master plan by International Planning Associates (IPA),[6][7][8] a consortium of three American planning and architecture firms made up of Wallace, Roberts, McHarg & Todd (WRMT – a group of architects) as the lead, Archi systems International (a subsidiary of the Howard Hughes Corporation), and Planning Research Corporation. The Central Business District of Abuja was designed by Japanese architect Kenzo Tange.[9][10][11] It replaced Lagos, the country's most populous city, as the capital on 12 December 1991.[12][13] Abuja's geography is defined by Aso Rock, a 400-metre (1,300 ft) monolith left by water erosion. The Presidential Complex, National Assembly,[14] Supreme Court and much of the city extends to the south of the rock. Zuma Rock, a 792-metre (2,598 ft) monolith, lies just north of the city on the expressway to Kaduna.[15]