Current population: 32.5 million
You know what's big in Japan? Tokyo. The 32.5 millionstrong metropolitan area is the thriving heart of the country and the most populous urban area in the world. Although Japan's overall birthrate is dropping and Tokyo's population peaked in 2004, people from across Japan still flock to the capital in search of good jobs. Every day, 2 million souls commute in and out of the metropolitan area on a vast network of always-on-time trains. The March 11 earthquake, tsunami and nuclear crisis in northeastern Japan scared some Tokyo residents away, but the exodus of 4,000 barely made a dent in the population of this high-density city.