a corrugated , thin walled plastic tank is ideally suited for subterranean storage and transmission of sewage and other materials . the tank has a bottom wall which terminates in a peripheral edge disposed in a horizontal plane . a corrugated , peripheral wall extends upward from the peripheral edge to an apex area . the corrguations of the peripheral wall include alternating crown and root portions . a peripheral rib extending circumferentially around the peripheral wall is defined by a plurality of rib portions which are disposed in the root portions of the corrugations . each rib portion has a rib outer portion which is substantially flush with the adjoining crown portions and rib side portions which extend from the rib outer portion into the adjacent root portion . an indentation area is formed by enlarging one of the root portions . an annular sleeve depends from the indentation area into the interior of the tank to provide a fluid access with the interior of the tank . an annular rim is disposed in the apex area surrounding another opening for providing access to the interior of the tank .