This book presents peer-reviewed articles from the 18th Conference of the Associated Research Centers for the Urban Underground Space (ACUUS 2023) held in Singapore from November 1 to 4, 2023. It highlights new opportunities and challenges in underground space use amid a re-focus on exploring and developing the underground space as a strategic resource and part of sustainable development. The new frontier presents unprecedented opportunities for simultaneously improving urban infrastructure, urban livability, and resilience. At the same time, special challenges exist in developing underground space--both from a human design perspective and in terms of complexity, 3D planning, system integration, technical, and life cycle costs. Equally important is the need to move from the creation of underground space as a haphazard, last resort solution to a well-planned, and integrated use of underground space that can serve the needs of society for future generations as well as our own. As the world emerges from the COVID-19 pandemic and is faced with the many challenges in climate resilience, the works presented in this book provide a timely opportunity and an excellent forum for engaging discussions and exchanges among planners, designers, engineers, researchers, and policy makers.