Events that are referred to as geysers have been observed in storm-water or combined sewer systems and are associated with jets of water rising through manholes to a considerable distance above the ground surface. Visual observations suggest that air may be a significant component of the jet. The mechanisms of geyser occurrence have been previously assumed to originate in inertial oscillations that force water up through vertical ventilation shafts. Recent laboratory investigations indicate that geyser formation is associated with the release of trapped air pockets through partially filled vertical shafts. Pressure data from a storm-water tunnel subject to infrequent geyser events is presented to indicate that measured piezometric heads adjacent to the ventilation shaft never increase to levels approaching the ground surface during a geyser event suggesting that air interactions must be an important part of the process. It is concluded that system design to avoid geyser formation must include the consideration of trapped air within the tunnel system.