The capital costs of underground hydrogen storage facilities are an important input for energy system models that assess the role that different technologies play in decarbonizing the energy sector. This report describes the methodology and assumptions used to estimate the cost of building new underground storage facilities and retrofitting existing underground natural gas facilities for hydrogen service. Cost estimates for three types of underground hydrogen storage facilities were modeled: depleted oil and gas reservoirs, aquifers, and salt caverns. The study collects and analyzes data from various sources, including public databases, published papers, and EPRI and LCRI cost studies, to estimate the capital costs of these technologies. The results also present spatial data on the availability and location of existing natural gas storage in the United States, which could be retrofitted to store hydrogen as well as scenarios for potential new hydrogen storage capacity.