Gihon Spring, lit blue underwater, is located beneath the City of David and is the starting point of both the 18th century BC Canaanite Tunnel and the 8th century BC Hezekiah's Tunnel.
Gihon Spring features prominently in the Bible. This natural spring ("Gihon" means "gushing") is the only reliable source of fresh water in Jerusalem. Without it, the Canaanites would not have built Jerusalem in the first place. Instead, they built the city and the Canaanite Tunnel to irrigate their crops in the Kidron Valley with Gihon Spring's water.
A thousand years later, King Hezekiah of Judah dug another tunnel, now named after him, to redirect the water from Gihon Spring to the Pool of Siloam and deny it to the besieging Assyrian army outside Jerusalem (see Hezekiah's Tunnel).
But Gihon Spring is best known in the Bible as the place where a 10th century palace coup was thwarted. Upon being informed that his fourth son Adonijah has declared himself king, the old and ailing King David ordered Nathan the prophet and Zadok the priest to take Solomon, his second son with Bathsheba, and anoint him king at the Gihon Spring, as detailed in 1 Kings 1.