Hydrogen is emerging as a crucial energy carrier in the transition toward a sustainable and low carbon energy system and its large-scale storage is essential for balancing supply and demand, particularly when integrating renewable energy sources. Among the various storage solutions, underground hydrogen storage (UHS) in depleted reservoirs stands out as a promising option due to their proven containment capabilities and the advantage of existing infrastructure that can potentially be repurposed for hydrogen injection and withdrawal. This study begins by exploring the critical factors that influence the feasibility of UHS, including geological and reservoir characteristics, various trapping mechanisms that enhance hydrogen retention, the behavior of hydrogen in subsurface conditions, rock-fluid interactions, and the potential risks associated with long-term storage, while also drawing insights from real-world case studies to provide practical context.