Most previous studies of hydrogen fuel cell vehicle (HFCV) releases in garages have focused on residential garages. However, hydrogen releases in large underground parking garages can be completely different with more concerns about public safety. In the present work, unintended hydrogen releases were studied in a scaled-down underground parking garage model to fill the knowledge gap concerning hydrogen dispersion in a large confined space. Helium was used in place of hydrogen for safety reasons. Gas sensors were used to measure the helium concentrations along the ground and the ceiling. The nearfield concentration distributions were studied to show that the leak jet was strongly affected by the buoyancy, so the gas concentration was very low along the floor with a flammable gas mixture only appearing right under the leaking car. The gas plume driven by the buoyancy reached the ceiling and spread in all directions resulting in a flammable layer along the ceiling. The effects of neighboring cars and walls were also investigated to show that the obstacles affected the nearfield gas dispersion but did not significantly affect the farfield concentration distributions. The concentration increased linearly with the release flow rate for the flow rates in this study. The gas dispersion during the release showed self-similar characteristics, which could be used for leak detection. This database will be useful for model validation for modeling HFCV releases in large underground parking garages and also provides guidelines for gas sensor placement in large confined spaces. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.