Jamieson, Fausset and Brown on 2 Chronicles 32:30

30. stopped the ... watercourse of Gihon, and brought it ... to the west side of the city, &c.--(Compare 2Ki 20:20). Particular notice is here taken of the aqueduct, as among the greatest of Hezekiah's works. "In exploring the subterranean channel conveying the water from Virgin's Fount to Siloam, I discovered a similar channel entering from the north, a few yards from its commencement; and on tracing it up near the Mugrabin gate, where it became so choked with rubbish that it could be traversed no farther, I there found it turn to the west in the direction of the south end of the cleft, or saddle, of Zion, and if this channel was not constructed for the purpose of conveying the waters of Hezekiah's aqueduct, I am unable to suggest any purpose to which it could have been applied. Perhaps the reason why it was not brought down on the Zion side, was that Zion was already well-watered in its lower portion by the Great Pool, 'the lower pool of Gihon.' And accordingly Williams [Holy City] renders this passage, 'He stopped the upper outflow of the waters of Gihon, and led them down westward to the city'" [Barclay, City of the Great King]. The construction of this aqueduct required not only masonic but engineering skill; for the passage was bored through a continuous mass of rock. Hezekiah's pool or reservoir made to receive the water within the northwest part of the city still remains. It is an oblong quadrangular tank, two hundred forty feet in length, from one hundred forty-four to one hundred fifty in breadth, but, from recent excavations, appears to have extended somewhat farther towards the north.