By Jonathan Saul and Stephen Farrell
JERUSALEM/LONDON (Reuters) -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 Israeli military accused Hamas on Friday of using Gaza's main hospital, Al Shifa, as a shield for tunnels and operational centres.
"Hamas has turned hospitals into command and control centres and hideouts for Hamas terrorists and commanders," Rear Admiral Daniel Hagari, Israel's chief military spokesman, said.
He showed photographs, diagrams and audio recordings that, he said, showed how Hamas was using hospitals to hide command posts and entry points into the tunnel network. "Hamas terrorists operate inside and under Shifa hospital and other hospitals in Gaza," he said.
It was not possible to verify Hagari's statements. Hamas official Ezzat El-Reshiq said there was "no basis in truth to what was reported by the enemy army spokesman".
The United States believes Israel's special forces will face an unprecedented challenge battling militants while trying to avoid killing hostages held below ground, a U.S. official said.
U.S. Defense Secretary Lloyd Austin noted that Iraq's nine-month-long battle to retake 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 hostilities in 2021, Hamas's leader in Gaza, Yehya Al-Sinwar, said: "They started saying they destroyed 100 km of Hamas tunnels. I am telling you, the tunnels we have in the Gaza Strip exceed 500 km. Even if their narrative is true, they only destroyed 20% of the tunnels."
There has been no corroboration of the comment by Sinwar, who is thought to be hiding underground ahead of an expected Israeli offensive.
But the estimate of hundreds of kilometres is widely accepted by security analysts, even though the blockaded coastal strip is only 40 km (25 miles) long.
With Israel in control of Gaza's air and sea access and 59 km of its 72 km land borders - with Egypt 13 km to the south - tunnels provide one of the few ways for Hamas to bring in weapons, equipment and people.
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 that 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".
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 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."
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 co-ordination 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 El Arish, but they slowed to a near-halt after the 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.
Hamas is thought to have begun digging tunnels in the mid-1990s, when Israel granted Yasser Arafat's Palestine Liberation Organization 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 military bases and monitoring devices make it harder to get anything in from Jordan.
Tunneling 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 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 commercial tunnels under the Rafah border.
These were around three feet (one metre) wide and used winch motors to haul goods in hollow barrels.
One Rafah tunnel operator, Abu Qusay, said a half-mile tunnel took three to six months to dig and could yield 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.
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 and destroying tunnels.
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 they face an enemy that has regrouped and learned from previous operations.
"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."
(Reporting by Jonathan Saul in Jerusalem and Stephen Farrell in London; Additional reporting by James Mackenzie in Jerusalem, Phil Stewart in Washington, Nafisa Eltahir and Ahmed Mohamed Hassan in Cairo; Editing by Janet Lawrence and Alison Williams)