ITACUS Co-chair Han Admiraal gave a keynote at the 6th International Symposium on tunnels and shafts in soils and rocks organised by ITA Member Nation representative for Mexico, AMITOS. Han spoke on the Urban Challenges & Opportunities facing the world's cities. He introduced two fundamental principles: the need for underground space planning and the need for multiple and integrated uses of underground spaces. He also gave three key messages: (1) business as usual no longer suffices; (2) we need to mitigate and adapt to climate change; and (3) underground space is part of that process of mitigation and adaptation. The subsurface is seen as a solution for alternative energy sources or storage of carbon dioxide. We see a scramble for underground spaces for various uses that need to be planned, coordinated, and regulated to prevent urban underground chaos. This is why it's essential that ITACUS, together with ISOCARP and other organisations, pursue underground urbanism.