Abuja () is the capital city of Nigeria located in the centre of the country within the Federal Capital Territory (FCT). It is a planned city, designed as a new national capital city to replace Lagos, and was built mainly in the 1980s, replacing the country's most populous city of Lagos as the capital on 12 December 1991. Abuja's geography is defined by Aso Rock, a monolith left by water erosion. The Presidential Complex, National Assembly, Supreme Court and much of the city extend to the south of the rock. Zuma Rock, a monolith, lies just north of the city on the expressway to Kaduna.