The grandest expression of the world's population growth is the word "megacity." In them, people and ideas clash: The ancient collides with the modern; secular with religious; global with local. In Karachi, Pakistan, those forces can be seen in the story of a single piece of real estate.

The Stuxnet computer worm successfully damaged centrifuges at a nuclear facility in Iran. Now, officials responsible for defending U.S. infrastructure fear that Stuxnet may have provided a blueprint for adversaries who may want to sabotage industrial operations in this country.

For decades, Syrian President Bashar Assad and his late father Hafez Assad did not allow any real politics in Syria. But the protests are teaching them how to organize.

While much of the world worries about how to sustain 7 billion people, in many countries, low birthrates are the more pressing problem. From Germany and Russia to Japan and South Korea, leaders are desperate to reverse a trend of lower birthrates.