ITV News Correspondent Robert Moore reflects on the rare access he was given to a complex network of tunnels used by Hamas leaders in 2009, which are still being used today
Hamas has a tiny fraction of the resources of the Israeli Defence Forces (IDF).
The group, after all, has been under siege for 16 years.
The weapons their fighters used to have to be manufactured in improvised factories, or they must be smuggled in from Egypt.
Out in the open, the Izz ad-Din al-Qassam Brigades - Hamas' military wing, responsible for last weekend's atrocities - are no match for the heavy armour and high-tech elite units of the IDF.
But they won't be meeting the Israelis in the open, but in the crowded backstreets of Gaza City and the Jabalya refugee camp.
And that will be the terrain that gives Hamas some ability to inflict heavy casualties on the IDF.
It is a world of twisting alleys and rubble from years of aerial bombardment.
It is a battered, maze-like urban landscape with which the militants are intimately familiar and where they will make their last stand.
There is another feature that favours Hamas that I have experienced on a reporting trip to Gaza.
It is the astonishingly complex network of tunnels.
Few people outside of Gaza have seen it, but from the sandy soil on which the destitute enclave sits, Hamas tunnelers have carved an underground labyrinth.
I was taken down some of the tunnels in 2009. My Palestinian contacts wanted to showcase their engineering ingenuity.
Some of the most secretive tunnels, used by Hamas leaders, I was unable to tour, and even my guides appeared to be nervous when we passed their concealed entrances.
Dozens of the tunnels snake their way under the border to Egypt, facilitating an industrial-scale smuggling operation.
Others are used to store weapons and explosives, and there are even subterranean factories.
It is likely that Hamas leaders use their own dedicated tunnels as safe spaces, away from Israeli drones and electronic intercepts, to plan operations.
It is impossible to know for sure, but it is probable that this is also where some of the Israeli and foreign hostages are currently being held.
From what I saw, it is difficult to imagine how even Israel's legendary special forces could pull off a hostage rescue down in these tunnels.
The tunnels that I went down were surprisingly sophisticated, with a complex arrangement of pulleys and carts, a lighting system, with many high enough to run though in a crouching position.
Military expert Colonel Mark Cancian fought in urban battles in Iraq. He told ITV News drones and specialist munitions will pay a key role in an IDF invasion of Gaza.
They can be located using ground radar and other sensors, which the Israelis often use, but there are so many tunnels that there is redundancy.
Destroy one, the smugglers use another. Destroy a network, and the tunnellers start again.
So the invasion of Gaza won't replicate the US assault on Fallujah, or the Iraqi capture of Mosul from ISIS.
This will be a war, yes, partly conducted in the air and on the ground, like in those campaigns.
But to defeat Hamas, the war must also be waged underground.
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