The behavior of stormwater systems can be greatly altered by the presence of air pockets within the pressurized portion of the flow. While rapid filling conditions in stormwater storage tunnels can result in air pocket entrapment, the mechanisms for this phenomenon have not been studied systematically to date. This paper summarizes an investigation conducted at the University of Michigan on an experimental setup that reproduces the essential features of a below grade storage tunnel. Various filling scenarios were tested in which the filling rate, initial water depth and ventilation configurations were varied. Different and previously unreported mechanisms for air pocket entrapment were identified. For the conditions investigated in the experiments, the formation of air pockets was relatively common.