An older Canaanite tunnel had been very vulnerable to attackers, so, under threat from the Assyrian king Sennacherib, Hezekiah sealed up the old outlet of the Gihon Spring and built the new underground Siloam tunnel in place of the older tunnel (2Chr 32:2 -4). During this period the Pool of Siloam was sometimes known as the Lower Pool (Isa 22:9 ), as opposed to a more ancient Upper Pool (2Kings 18:17;Isa 7:3 ) formerly fed by the older Canannite tunnel.

The Pool of Siloam was built during the reign of Hezekiah (715–687/6 BCE), to leave besieging armies without access to the spring's waters. The pool was fed by the newly constructed Siloam tunnel.

Wikipedia