Sim City, the popular computer game, taps into the fact that everyone has an idea of how they'd design a city if they just had the chance. Two brand-new cities currently being built in the Persian Gulf region represent two different responses to this challenge. Waterfront City in Dubai (right) is planned by Reinier de Graaf and Rem Koolhaas of the Office for Metropolitan Architecture (OMA). Projected to have a population of 92,000, it is the centerpiece of Waterfront, a massive development that will eventually house 1.5 million people. This is actually more than the entire current population of Dubai, but new cities require nothing if not ambition: empty desert one day, my condo looking out on the water the next.