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 US official said.
US defence secretary Lloyd Austin noted that Iraq's nine-month-long battle to retake the city of Mosul from Islamic State might prove to have been easier than what awaits the Israelis - likely to be "a lot of IEDs (improvised explosive devices), a lot of booby traps, and just a really grinding activity".
Even though Israel has invested heavily in tunnel detection - including a sensor-equipped underground barrier it called an "iron wall" - Hamas is still thought to have working tunnels to the outside world.
After the last round of hostilities in 2021, Hamas's leader in Gaza, Yehya Al-Sinwar, said: "They started saying they destroyed 100kms of Hamas tunnels. I am telling you, the tunnels we have in the Gaza Strip exceed 500kms. Even if their narrative is true, they only destroyed 20 per cent of the tunnels."
Hostage witness
There has been no corroboration of the comment by Sinwar, who is thought to be hiding underground ahead of an expected Israeli ground offensive.
But the estimate of hundreds of kilometres is widely accepted by security analysts, even though the blockaded coastal strip is only 40km (25 miles) long.
With Israel in full control of Gaza's air and sea access and 59km of its 72km land borders - with Egypt 13km to the south - tunnels provide one of the few ways for Hamas to bring in weapons, equipment and people.
It looked like a spider's web, many, many tunnels.
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While it and other Palestinian groups are secretive about their networks, recently released Israeli hostage, 85-year-old Yocheved Lifshitz, said: "It looked like a spider's web, many, many tunnels," adding: "We walked kilometres under the ground."
Hamas believes that with Israel's overwhelming aerial and armoured military superiority, tunnels are a way to cut some of those advantages by forcing Israel's soldiers to move underground in cramped spaces the Hamas fighters know well.
An Israeli military spokesperson said on Thursday: "I won't elaborate on the number of kilometres of tunnels, but it is a high number, built under schools and residential areas."
Urging the United Nations Security Council to intervene, Palestinian president Mahmoud Abbas has called for an immediate cessation of "aggression" on Gaza and moves toward "a political solution instead of military and security solutions".
Underground City
Israeli security sources say Israel's heavy aerial bombardments have caused little damage to the tunnel infrastructure with Hamas naval commandos able to launch a seaborne attack targeting coastal communities near Gaza this week.
"Although we have been attacking massively for days and days, the (Hamas) leadership is pretty much intact, as is the ability to command and control, the ability even to try and launch counter attacks," said Amir Avivi, a former brigadier general whose senior positions in the Israeli military included deputy commander of the Gaza division, tasked with tackling tunnels.
"There is a whole city all over Gaza underneath with depths of 40-50 metres. There are bunkers and headquarters and storage and of course they are connected to more than a thousand rocket launching positions."
Other sources estimated depths of up to 80 metres.
One Western security source said: "They run for miles. They are made of concrete and very well made. Think of the Viet Cong times 10. They have had years and lots of money with which to work with."
Another security source, from one of Israel's neighbouring countries, said Hamas' tunnels from Egypt remain active.
"The supply chain is still intact these days. The network involved in facilitating coordination are some Egyptian military officers. It is unclear if there is knowledge of this by the Egyptian army," he said.
A small number of narrower, deep, smuggling tunnels were still operating until recently between Egypt and Gaza, according to two security sources and a trader in the Egyptian city of El Arish, but they had slowed to a near-halt since the Israel-Hamas war started.
Egyptian officials did not immediately respond to a request for comment. On Wednesday, Egyptian president Abdel Fattah al-Sisi said while inspecting military units in Suez that the army's role was to secure Egyptian borders.
Long game
Hamas was created in Gaza in 1987 and is thought to have begun digging tunnels in the mid-1990s, when Israel granted Yasser Arafat's Palestine Liberation Organisation some degree of self-rule in Gaza.
The tunnel network is a key reason why Hamas is stronger in Gaza than in the Israeli-occupied West Bank, where Israel's settlements, military bases and monitoring devices make it harder to get anything in from Jordan.
Tunnelling became easier in 2005 when Israel pulled its soldiers and settlers out of Gaza, and when Hamas won power in a 2006 election.
Shortly afterwards, Hamas's military wing, the Izz el-Deen al-Qassam Brigades, captured Gilad Shalit and killed two other Israeli soldiers after burrowing 600 metres to raid the Kerem Shalom base on the Gaza border.
A year later, Hamas used tunnels in Gaza to launch a military strike against the forces of Arafat's successor as PLO leader, Mahmoud Abbas.
Although the military tunnels remained off-limits to outside eyes, during that era Gaza smugglers would show off their scarcely concealed commercial tunnels under the Rafah border.
These were around three feet (one metre) wide and used winch motors to haul goods along the sandy tunnel floors in hollowed-out petrol barrels.
One Rafah tunnel operator, Abu Qusay, said a half-mile tunnel took three to six months to dig and could yield profits of up to $100,000 a day. The most profitable item was bullets, bought for $1 each in Egypt and fetching more than $6 in Gaza. Kalashnikov rifles, he said, cost $800 in Egypt and sold for twice that.
In 2007, the military wing is thought to have brought its commander Mohammed Deif into Gaza through a tunnel from Egypt. Deif was the mastermind behind Hamas's deadly October 7th attack into Israel, which killed 1,400 people and hostages were taken.
Tunnel hunting
Professor Joel Roskin, a geomorphologist and geologist with Israel's Bar-Ilan University said it was difficult to map the tunnel network accurately from the surface or space, adding highly classified information was essential for 3D mapping and imagery visualisation.
Among the elite units tasked with going underground is Yahalom, specialist commandos from Israel's Combat Engineering Corps known as the "weasels", who specialise in finding, clearing and destroying the tunnels.
Earlier this week Israeli prime minister Benjamin Netanyahu visited Yahalom fighters, telling them: "I rely on you, the people of Israel rely on you."
Israeli sources said what awaits them is formidable, and they faced an enemy that has regrouped and learned from previous Israeli operations in 2014 and 2021.
'Booby traps'
"There are going to be a lot of booby traps. They have thermobaric weapons that they didn't have in 2021, which are more lethal. And I believe they acquired a lot of anti-tank weapon systems that are going to try to hit our APCs (armoured personnel carriers), tanks," said Amnon Sofrin, a former brigadier general and former commander of the Combat Intelligence Corps.
Sofrin, who was also previously head of the intelligence directorate with Israel's Mossad spy agency, said Hamas would also be trying to kidnap soldiers.
Daphne Richemond-Barak, professor at Israel's Reichman University and author of the book Underground Warfare, said the conflicts in Syria and Iraq had changed the situation.
"What the IDF (Israeli military) is likely to face inside the tunnels is also all of the experience and all of the knowledge that has been gained by groups like Isis (Islamic State) and has been ... passed on to Hamas."
