One of the underground tunnels dug by Herod the Great that were connected to the system of cisterns.
The tunnels were expanded by Jewish Zealots during the two revolts against Rome (A.D. 66-70 and 132-135).
Netzer, Ehud. "Jewish Rebels Dig Strategic Tunnel System." Biblical Archaeology Review, vol. 14, no. 4 (July/August, 1988):18-33.
For a Map and a brief description of the Herodium Click Here.