4. Delhi

Population: 22,200,000
Country: India

Delhi is northern India's largest city. One part of it, known as New Delhi, is officially designated the capital of India, but the names are often used interchangeably. Delhi is said to be one of the oldest existing cities in the world, along with Damascus and Varanasi. Legend estimates it to be over 5000 years old. Over the millennia, Delhi is said to have been built and destroyed 11 times. Delhi is as flat as a pancake. River Yamuna flows down the eastern side of the city and the Aravalli Hills form a wide but low arc across the west. On the west bank are crowded and congested Old Delhi and, to the south, the broad, tree-lined avenues of New Delhi, built by the British to rule their empire. The rest is an endless low-rise sprawl of suburbia and slums, with southern Delhi generally somewhat wealthier and the western reaches rather poorer.