Various rapid filling scenarios lead to the formation of large discrete air pockets within a below ground storage tunnel system. The release of trapped air pockets through a vertical shaft can potentially cause a geyser event in which untreated wastewater returns to the ground surface. It is important to the numerical modeling and design efforts of these expensive systems to estimate the potential for such events. However, little is known about the behavior of discrete air pockets once they are trapped within tunnel systems. Initial laboratory experiments are performed to measure the direction and velocity of air pockets as they migrate within nearly horizontal systems. A simple methodology for predicting the migration of air pockets is proposed and compared with the experimental data. Next, a diameter expansion within the vertical shaft is verified as a possible design modification to mitigate geyser events. Experimental variables such as the ratio of expansion diameters and the vertical location of the expansion are investigated to optimize the proposed design. The scalability of experimental results is explored by performing tests on a 0.095 m diameter tunnel as well as a 0.203 m diameter tunnel. Some qualitative conclusions are presented and discussed from the preliminary experimental results.