Abstract: An underground storage tank includes an outer wall having integrally formed ribs, the ribs being closed off by a layer of material applied directly over the space defined created by the ribs and attached to the outer wall, a layer of plastic film or other annular material through which liquid may flow placed over portions of the outer wall between the ribs and extending past the shoulders of the ribs but not extending completely over the material covering the opening of the rib, and an inner wall formed over the annular material and attached to the portion of the material covering the opening provided by the rib that is not covered by the annular material. In this way, the inner wall is tied and preferably chemically and mechanically bonded to the outer wall through the material covering the opening of the rib.