The Hamas tunnel city beneath Gaza: a hidden frontline for Israel

What lies in wait for Israeli ground troops in Gaza, security sources say, is a Hamas tunnel network hundreds of kilometres long and up to 80 metres deep, described by one freed hostage as "a spider's web" and by one expert as the "Viet Cong times 10".

The Palestinian Islamist group has different kinds of tunnels running beneath the sandy 360-sq-km coastal strip and its borders - including attack, smuggling, storage and operational burrows, Western and Middle East sources familiar with the matter said.

The United States believes Israel's special forces will face an unprecedented challenge having to battle Hamas militants while trying to avoid killing hostages held below ground, a U.S. official said.