2 Chronicles 32:4

All the fountains. The principal spring that Hezekiah stopped was the Gihon (v. 30), the present Virgin’s Spring, on the south slope of the hill on which the Temple was built. It was in a cave outside the city wall, and its waters originally flowed into the Kidron Valley, where it would have been a source of supply to the Assyrian invaders.

The Jebusites, before the capture of Jerusalem by David, had cut a conduit, bringing the waters from the spring to a place where the supply could be reached through a shaft from within the city (see 2 Sam. 5:8). Later an aqueduct let this water into the Old, or Lower, Pool (Isa. 22:9, 11). Hezekiah made a new aqueduct, the Siloam tunnel (see 2 Chron. 32:30; see on 2 Kings 20:20), that carried the waters to a new reservoir, the Pool of Siloam (see Neh. 3:15; John 9:7), and probably built a new wall that placed the new pool inside the city (see on v. 5). Thus the fountains outside the city were diverted for use within the walls.