This structure has been interpreted as a cistern or a tank owing to its rectangular shape, and more significantly the opus signinum that covers the inside of the structure. The construction action of the cistern/tank (not documented separately) cuts the cocciopesto floor 5585, and the concrete for the cistern. Cleanly fills the cut in the floor, abuting 5585. The cistern has been interpreted as in phase with the opus reticulatum wall 5302. Only three of the original limits of the wall remain, as the southern limit was cut by wall 5348. Wall 5348 has not been carefully documented or studied.