This ritual bath (miqveh) is located just south of the Double and Triple Gates. It is one of over forty ritual baths that Benjamin Mazar discovered in his excavations south of the Temple Mount. These baths were used by worshipers who purified themselves prior to entering the temple area — via the Double or the Triple Gates.
Note the steps leading down to lowest level. On the steps, right of center, is the remnat of a very low wall. Possibly one descended into the bath on one side of the "wall" in an impure state, and then ascended from the bath on the other side in a "pure" state. Initially the bath was probably covered, but the roof has disappeared.
Some Christian commentators have wondered if possibly some of these ritual baths may have been used by the early Jewish Christians for baptismal purposes (Acts 2:41) on the day of Pentecost — when 3,000 persons were added to the community of believers. [This possible usage is somewhat speculative]