Named after its English discoverer, this is a rock-cut sloping tunnel leading to a vertical shaft through which a bucket could be dropped by rope into a pool fed by the Gihon Spring. The horizontal crack above the shaft is the frontier between the porous meleke limsestone above and the impermeable mizzi ahmar dolomite below. Rain-water percolating through the limestone flowed along the surface of the dolomite to create the horizontal tunnel, which continues above the shaft to exit on the slope outside. A fissure in the dolomite attracted water downwards creating a sink-hole, which is more than 40,000 years old. These natural features were skillfully enlarged by the Jebusites in order to provide secure access to water when under siege. The entrance to the tunnel was well within the C18-C6 city wall further down the hill. Either the horizontal tunnel or the vertical shaft could be the sinnor ‘water-shaft’ by which Joab gained entrance to the city for David (2 Sam. 5:8; 1 Chr.11:6).
Source: The Holy Land by Jerome Murphy- O’Connor