2007 is the year that the world's population becomes more urban than rural for the first time. This exciting series explores some of these global cities, and looks at how the challenges of urbanisation, globalisation, citizenship and sustainability are being met and answered.

Founded by Spanish conquistadors on the ruins of Tenochtitlan, Mexico City is the capital of Mexico, the second-largest city in the world, and one of the most polluted. Its raid growth has been largely unplanned, leading to monumental urban challenges. Mexico City's potential is enormous, and major initiatives are being put in place to tackle the city's problems for the 21st century.