As our global population grows each day, it is apparent that we must find new ways to provide space for habitation and the services necessary to support large urban areas. It is no longer advisable nor acceptable to continue to expand our cities on the surface, encroaching on valuable agricultural land and open space. We must also find ways to be more energy efficient in every aspect of our societies. Effective use of underground space can provide the space necessary to accommodate larger populations and the services necessary for their support in existing urban areas we can also find new ways to utilize underground facilities to improve urban efficiency and function. Underground technology has improved dramatically in the last two decades and continues to evolve to meet a great variety of applications. The confluence of available technology, economic feasibility, and greater acceptance of underground solutions with recognition of the need to change the ways we build and use out urban areas and their surrounding environment indicates that we are witnessing only the beginning of appropriate and innovative use of underground space future generations will indeed 'Think deep'.