Mumbai is the capital city of the Indian state of Maharashtra. It is the most populous city in India and the ninth most populous agglomeration in the world, with an estimated city population of 18.4 million. Along with the neighbouring regions of the Mumbai Metropolitan Region, it is one of the most populous urban regions in the world and the second most populous metropolitan area in India, with a population of 20.7 million as of 2011. Mumbai lies on the west coast of India and has a deep natural harbour. In 2009, Mumbai was named an alpha world city. It is also the wealthiest city in India, and has the highest GDP of any city in South, West, or Central Asia. Mumbai has the highest number of billionaires and millionaires among all cities in India.