Human population growth seems inevitable if problematic. As the number of humans on the planet continues to increase, it's likely more and more people will crowd into dense urban areas with over 100,000 people per square mile. What will life be like? And what does that do to the planet? Here's one greentechie's vision of how sustainable megacities could be built and, well, sustained. He has one idea that future generations may really dig, getting below the surface literally.