It has long been known that the Milk River A Gas Pool is located downdip of an aquifer which outcrops in Southern Alberta. In the past numerous suggestions have been made explaining how this unconventional trap of gas below water could exist. These suggestions were without theoretical or practical application. In this paper a theoretical model of the trapping mechanism is described and provides a clear explaination of the phenomenon. It is shown that buoyancy forces are not dominant until pore throats become larger and interfacial tension forces are less than buoyancy. A pressure-depth plot for the Milk River provides additional understanding of the hydrodynamic state of the water and gas in the pool. Application of the theoretical model also provides a basis for understanding edge wells whose production and recoveries are influenced by the aquifer.