Underground Science City

When you cannot grow up, the only alternative could be to grow down.  Check out what Singapore is considering: 

"But as one of the world’s most crowded cities, and with projections for 1.5 million more people in the next 15 years, Singapore’s options are as limited as its space. So Singapore is considering a novel solution: building underground to create an extensive, interconnected city, with shopping malls, transportation hubs, public spaces, pedestrian links and even cycling lanes."