Islamabad is the capital of Pakistan and the tenth largest city in the country. Located within the Islamabad Capital Territory, the population of the city has grown from 100,000 in 1951 to 1.7 million in 2011. The greater Islamabad-Rawalpindi Metropolitan Area is the third largest conurbation in Pakistan with a population of over 4.5 million inhabitants. Islamabad is located in the Pothohar Plateau in the northern part of the country, within the Islamabad Capital Territory. The region has historically been a part of the crossroads of Punjab and Khyber Pakhtunkhwa with Margalla pass acting as the gateway between the two regions. The city was built during the 1960s to replace Karachi as Pakistan's capital. Islamabad is a well-organized city divided into different sectors and zones. It is regarded as the most developed city in Pakistan. It was ranked as a Gamma world city in 2008. The city is home to Faisal Mosque, the largest mosque in South Asia and the sixth largest mosque in the world. Islamabad has the highest literacy rate in Pakistan and is home to the some of the top-ranked universities in Pakistan, including Quaid-i-Azam University, Pakistan Institute of Engineering & Applied Sciences and the National University of Sciences and Technology. Allama Iqbal Open University in Islamabad is one of the world's largest universities by enrollment.