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...
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.
(photo from http://www.bib-arch.org/e-features/hezekiahs-tunnel.asp)
