From Wikipedia,
Hezekiah's Tunnel is a tunnel that was dug underneath the City of David in Jerusalem in ancient times. Its popular name is due to the most common hypothesis of its origin, namely that it dates from the reign of Hezekiah of Judah (late 8th and early 7th century BCE) and corresponds to the waterworks mentioned in 2 Kings 20:20 in the Bible
The tunnel, leading from the Gihon Spring to the Pool of Siloam, was designed as an aqueduct to provide Jerusalem with water during an impending siege by the Assyrians, led by Sennacherib. The curving tunnel is 533 m long, and by using a 30 cm (0.6‰) gradient altitude difference between each end, conveyed water along its length from the spring to the pool.
According to the Siloam inscription, the tunnel was excavated by two teams, one starting at each end of the tunnel and then meeting in the middle. The inscription is partly unreadable at present, and may originally have conveyed more information than this. It is clear from the tunnel itself that several directional errors were made during its construction. Recent scholarship has discredited the idea that the tunnel may have been formed by substantially widening a pre-existing natural karst.
The difficult feat of making two teams digging from opposite ends meet far underground is now understood to have been accomplished by directing the two teams from above using sounds generated by hammering on the solid karst through which the tunnelers were digging.

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