A back pack for an underwater breathing gas tank is provided. The back pack includes a tank mounting structure having a front surface facing away from an underwater breathing gas tank when an underwater breathing gas tank is operably mounted to the tank mounting structure. The tank mounting structure defines a recess. The front surface defines an opening of the recess. The tank mounting structure defines a bottom surface of the recess opposite the opening in the front surface. The back pack includes a strap and a buckle. The buckle has a latched state for securing the strap in a fixed position for securing an underwater breathing gas tank to the tank mounting structure. The buckle, when in a latched state, is positioned at least partially within the recess.