The importance of underground resources in addressing societal challenges such as climate change, population growth, and urbanisation is increasingly evident, as they play a crucial role in advancing sustainability goals. However, current underground development is often uncoordinated and reactive, following ad-hoc, “first-come, first-served”, or last-resort principles. This fragmented approach results in unsustainable management practices that overlook opportunities for synergies between underground uses. This chapter outlines synergistic relationships between various underground resource uses by drawing on real-world examples. We have identified three core principles that enable subsurface synergies: multifunctionality, circularity, and repurposing. By showcasing examples from around the globe, we illustrate how these principles can be integrated into legal planning frameworks, such as master plans, to promote sustainable underground development and ensure long-term functioning. To facilitate and implement subsurface synergies effectively, a holistic, interdisciplinary, and integrated approach to underground planning is essential. Researchers, policymakers, legislators, and practitioners must go beyond mere conflict resolution arising from competing underground uses to leverage synergies for long-term sustainable development above and below ground.