The tunnel. The impressive engineering feat that you see in front of you was probably built in the third quarter of the first century BC, when the Nabatean started building structures and there was a need to protect them from the flash floods common to the area, and to protect those walking through the Siq. The tunnel is 88 meters long and was cut in the rock to divert flood waters away from the Siq into Wadi Al Mudhlim and to Wadi Al Mataha. The water diverted through this tunnel helped to supply the Nymphaeum inside the city. Text from notice on site. I walked through the tunnel when coming from the other side, that series of pictures is elsewhere in the Petra gallery under Wadi Muthlim.