Cut 5891 was cut into the bedrock in the south of room 20 for the construction of a drainage feature along the southern extent of that room. This drainage feature was lined with opus mixtum walls on its west (SU5875), north (SU5889), and east (5890) sides with at least the northern side finished in a layer of opus signinum. The drainage feature was fed by capucchini drains from both the east and west. It is unclear if this drainage feature is interconnected with those in Room 21.