The housing problem in Hong Kong is critical. Studies estimate that the city of seven million will have to house another 600,000 people over the course of the next 30 years. With rapidly increasing urbanization rates, leading Chinese metropolises are speculating on fast and intelligent ways to manage population growth by creating additional housing within their existing borders. While some cities are growing taller and others are mulling developing rare and cherished park space, Hong Kong is taking a different approach. Officials and engineers have thought about something else: developing an extensive underground city.