The Mumbai slums where Danny Boyle shot Slumdog Millionaire are a world of surprises. There are the gaggles of uniformed schoolchildren running through unpaved streets, satchels bouncing on their skinny backs - they study only in the morning or afternoon so they can work for a living the rest of the day. There are the Dickensian factories and workshops you come upon as you stumble down all but lightless alleyways. There is the general feeling of security - despite their scary reputation among middle-class Mumbaikars, and the rarity of policemen, the slums are as safe for outsiders to walk through as most other neighbourhoods of the city. Most of all, there is the strange way that the slums seem much less grim once you are actually inside them.

The population of Mumbai is at least 21 million, according to the last, inadequate census. More than half of them live in the city's 2,000-odd slums. Here, the term "slum" is a technical one. It does not just mean a poor or rundown area. It signifies a shantytown that has encroached on otherwise unoccupied public or private land. The slums are vast because they are made up of the hand-built homes of the millions who have come from all over India seeking work and a better life. Small new slums quickly grow around construction sites for buildings or new motorways to house the workers putting them up. Others appear suddenly on railway land or in areas left derelict by the closing of mills and factories that were once the economic engine of the city.

Though some well-heeled Mumbaikars resent the way many foreigners seem overly concerned with the slums, rather than impressed by the city's new skyscrapers, it is impossible not to notice them: the road from the airport goes past one of the worst, its shanties lining both sides of the highway. You can smell it long before and after you see it. Moreover, some of the city's smartest areas are abutted by slums.