CITIES GROW: Tokyo [opposite] is by far the largest megacity, with 35 million people, but its population increases by less than 100 000 per year.

Other megacities, however, are expanding at a furious pace. Take São Paulo, which has added 13 million people since 1960, almost the entire population of Shanghai. Most of São Paulo’s expansion is at its fringes, with some areas growing by a whopping 10 percent per year.


When a city has more people, it needs more ways for them to get around. Shanghai added 119 kilometers of ­subway line in 2006, with another 200 km under construction. Even New York City’s venerable 371‑km system, which opened in 1904, is still adding new lines and new ­stations, ­including one under the planned Freedom Tower, on the former site of the World Trade Center.