Seven Billion

This month's issue of the National Geographic Magazine focuses on the fact that there will soon be seven billion people on the planet.

In the image above by Randy Olson, Kolkata's steaming streets are crammed with vendors, pedestrians, and iconic Ambassador taxis, throbbing with some 16 million people—and more pour in every day from small towns. In 1975 only three cities worldwide topped ten million. Today 21 such mega cities exist, most in developing countries, where urban areas absorb much of the globe's rising population.