An adjustable length storage tank sump for attachment to an underground storage tank is provided. The storage tank sump is formed from a body having a polygonal lower portion with a chamber, an interconnecting conical middle portion, and an upper portion having a plurality of formations and an opening disposed thereon. The formations are axially spaced along the longitudinal axis of the body so that a predetermined number of the formations can be selectively removed during installation, thereby reducing the length of the storage tank sump. A sealing surface is provided on each formation so that a single seal can be installed on the formation closest the opening. An attachment structure, which is interchangeable among the formations, removably engages the formation closest to the opening. A removable lid for covering the opening is provided, the lid engaging the attachment structure such that the lid sealingly contacts the seal.