Cylindrical underground storage tank having particularly shaped end caps

A cylindrical storage tank formed of glass fiber reinforced plastic and adapted to be installed underground in a horizontally extending position, the tank comprising a hollow cylindrical body and two end caps joined respectively to opposite end portions of the cylindrical body, the end caps being outwardly convex and generally spherical at their central portions but being frusto-conical adjacent their junctions with the cylindrical body, the radius of curvature of the spherical portions being substantially equal to what it would be for completely hemispherical end caps corresponding in radius to the radius of the cylindrical body and the frusto-conical portion of each end cap forming an angle adjacent the bottom of the tank with an underground support surface when the tank is installed horizontally underground, said angle substantially corresponding to the natural angle of repose of recommended backfill material such as sand or gravel, whereby voids otherwise likely to occur beneath the end caps if the end caps were completely hemispherical are eliminated.