This chapter highlights the crucial role of subterranean environments in fortifying cities against climate change, particularly flooding and extreme weather. Underground spaces house essential services like power distribution, water supply, and data centres, enhancing urban resilience during crises. Large underground caverns offer new opportunities for logistics, production, utilities, storage, and entertainment. Strategically deploying these spaces can significantly increase urban capacity and free up surface areas for diverse uses. However, building subterranean infrastructure presents challenges, including structural integrity, construction methodologies, logistical complexities, and geotechnical issues. Ensuring fire safety and integrating underground developments with existing urban areas require careful planning and execution. This chapter examines the growing trend of underground utilisation, outlining the opportunities, challenges, and essential considerations for leveraging subterranean spaces to boost urban resilience and expansion. It highlights exemplary long-term cavern development strategies, such as those in Hong Kong. Hong Kong's approach involves meticulous geological assessments, innovative construction techniques, and stringent safety protocols. By leveraging its unique topography and geology, Hong Kong has transformed its underground spaces into thriving hubs for commerce, recreation, and infrastructure. This case study serves as a model for cities worldwide, demonstrating the transformative potential of strategic underground development in fostering sustainable urban growth and resilience.