KEY POINTS:
Israel's ground offensive is reaching a critical stage - facing Hamas fighters on their chosen killing ground, the narrow, winding alleyways of Gaza City.
Despite the days of relentless air strikes, Israeli commanders admit the Islamist movement still has large quantities of weapons and up to 20,000 trained men to use in a bloody campaign.
The Hamas arsenal has been smuggled in through the intricate network of
tunnels that dip under the Egyptian border, a network that has also provided the economic lifeblood for the Palestinian territory suffering from severe and punitive sanctions imposed by Israel.
The fact that these tunnels have played a key role in keeping Hamas in
political and military power has made them not only targets of Israeli attacks but also a key issue in any ceasefire.
There are believed to be hundreds of tunnels criss-crossing the 14km-wide barren border between Gaza and Egypt along what has become known as the Philadelphi Corridor. Constructed over years and varying in depth and width, the tunnels have carried everything from rockets to
cattle.
Professor Efraim Inbar, director of Israel's Bar-Ilan University's Begin-Sadat Centre for Strategic Studies, believes the Israeli
Government bears a degree of responsibility for the existence of the tunnels.
"They did not take them seriously at first and did not invest enough money and resources to detecting and stopping them. The problem they now face is that they have to destroy all the main tunnels and then also make sure that they are not rebuilt in the future."
In the past the Israelis have considered a number of options to deal with the tunnels, including digging a moat flooded with seawater. The plan was dropped, however, after it became apparent it could contaminate Gaza's crumbling underground aquifer.
The Israelis have also asked the US to provide its Army Corps of Engineers to build an underground wall on the Egyptian side of the
Philadelphi Corridor. The Americans are said to have agreed in principle
although it is unclear whether the Egyptians had also given approval.
For the moment, the Israelis are trying to destroy the network with pulverising bunker-busting bombs acquired from the United States.
Nicholas Pelham, an analyst with the International Crisis Group, said closing the tunnels permanently would be a long undertaking. "Without
occupying a fairly broad stretch of territory, it is hard to see how you can maintain the closure of the tunnels long-term," he said.
The more immediate concern for Israel is how Hamas's armed wing, the Izz al-Din al-Qassam Brigades, have used the tunnels to prepare for battle. The militia is said to have sent thousands of members for training to
Iran and Lebanon, where they have drawn on the tactics used successfully by Hizbollah against the Israelis in 2006.
According to one Hamas commander, one of the lessons learned is to reduce the risk of taking return fire by not detonating missiles
on site. Fighters "dig tunnels and use lengths of detonation wire so they can launch missiles from a distance. So we lose a tube or a firing frame worth $10, not soldiers".
Abu Bilal, a commander of Islamic Jihad, which works independently from Hamas in Gaza, acknowledged the rocket attacks have been psychologically damaging for Israelis but have little military impact.
"We can't do anything but fire the rockets and hope they enter Gaza. We are praying for the tanks to come so we can show them new things. All our fighters wait for the chance to kill them."
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