Current Population: 18.8 million
Brazil's financial and cultural hub is a thriving city of 18.8 million people, making it one of the largest cities in the southern hemisphere. São Paulo's economy runs on global demand for its iron ore and offshore oil, while international banks and hedge funds are attracted to its strong currency. But the city has in recent years been unable to cope with the exploding population, resulting in inadequate housing, skyrocketing property prices and slums known as favelas. With 35% of its residents living in extreme poverty, the city's high crime rate doesn't come as much of a surprise.