Reporter Michael Geller writes that the theme of the Expo this year is "The Living Ocean and Coast":
"[T]he entire site is designed to form a model city that displays the low-carbon lifestyle we can expect in the future. Each of the theme pavilions demonstrates ways to achieve sustainable development by promoting an environmentally responsible use of the sea."
The exhibit proposes that desalinization will replace the need for maintaining a fresh water supply, and "Energy needs will be satisfied in part by biofuels made from seaweed and other marine resources and new bio-degradable plastics will be made from materials found in the sea."
Here's a promo video from Expo 2012 (which unfortunately doesn't include the underwater city):
