It is a large drainage tunnel or sewer that runs down the Tyropoeon Valley and once drained runoff and waste water from the city of Jerusalem.
[1] According to Leen Ritmeyer, the drain is mainly of Hasmonean age, with the exception of a bypass section near the southeast corner of the Temple Mount, which is Herodian.
According to Josephus, in the year 70 CE thousands of Jerusalemites took refuge from the Roman sacking of Jerusalem inside this water channel.
[9][10] The tunnel was rediscovered in 2007 by archaeologists Ronny Reich and Eli Shukron who were excavating the monumental stepped street built during Pilate's governorship and leading up from the southern city gates via the Pool of Siloam and up towards the Temple when they happened on the water channel.
[11][2][12] Their excavations have eventually made accessible much of the length of the road between the Pool of Siloam and the south-eastern corner of the Herodian Temple Mount.