For the first time in history, more than half the population of the world now lives in cities. The need to shelter and sustain this population without destroying the delicate balance of the environment is a major dilemma for the coming century. Bustling Mexico City has become a symbol of all that could go wrong with urban development. The city's air is so thick with smog that eight out of ten days are declared hazardous to human health. The city's center has sunk 30 feet in the last 100 years as the city depletes its once abundant underground water supply, squeezing the ground like a sponge.
Raw sewage flows through open canals, carrying disease to surrounding farmlands.
Journey to Planet Earth looks at what went wrong and at some of the community-based efforts to find solutions. Lying astride Europe and Asia, Istanbul, capital of three great empires, has long been a cosmopolitan city. Economic and political refugees from Turkey's outlying regions pour into this ancient city at the rate of 500,000 newcomers a year. Bisecting the city is one of the busiest waterways in the world, the Bosphorous. As shipping traffic increases, ferries and fishing boats share the narrow strait with tankers loaded with flammable cargo, with occasionally disastrous results.