"Why Hezekiah's Tunnel Has the Bends" (p.5): A geologist may have the answer
Now an Israeli geologist, Dan Gill, has done some research on the matter and has come up with some very plausible explanations.
Dan identifies two types of rock in the tunnel area -- limestone and dolomite. The former is fairly soft and porous, the latter comparatiively hard. It is rather interesting that this limestone consists of about 30% fragments of fossil shells and some coral, which means that Jerusalem, which is now about 700 metres above sea level, must have been beneath the ocean at some time in the past .