Cities of the world are growing fast and only a small part of that expansion is being planned strategically. In recognizing the dilemma, the Think Deep initiative of the Committee on Underground Space of the International Tunnelling and Underground Space Association (ITACUS) has been leading the discussion on how underground space can be harnessed more effectively in the planning and development of more sustainable urban cities for the future. As part of its strategy ITACUS has supported the establishment of Think Deep groups in member nations of the ITA as well as an international group of Think Deep Young Professionals.