As Israeli forces prepare to enter Gaza, a major part of the battle will be underground

If as anticipated,the IsraelDefenceForcespush into Gaza as part of a ground offensive in retaliation for thedeadly attack on Israel by Hamas, they will be fighting in a densely populated strip that is home to more than twomillion people.

But what some call the "city below the city" may prove to be the biggest challenge.

The sophisticated network of tunnels constructed by militant group Hamas criss-crosses the Palestinian enclave, providing sheltered corridors to run weaponswhile bunkers deep underground house Hamas's command centres.

For years, the Iranian-backed militant group which has been classified asa terrorist organization by several countries, including Canada has been spending millions of dollars fortifying thetunnels that it claims stretch for 500 kilometres.

Hamas has saidit could be holding some of the 200 hostages seized in the Oct. 7 attacks, including Israeli soldiers, underground.The Israeli government says morethan 1,400 people werekilled in the attacks.

The tunnels, which Israel has been targeting through airstrikes, are oftenhard to detect, according to experts.

Israeli forces tasked with clearing them will be vulnerable to ambush and booby trapswhile the Palestinians living above them are at great risk of being caught up in a war thathas already killed more than 4,000people living in Gaza and made more than a million homeless, according to Palestinian officials and the United Nations.

"If you blow up a tunnel under major civilian infrastructure, it will damage the infrastructure above it," saidRaphael Cohen, a senior political scientist in the Washington, D.C., office of theRand Corporation, a think-tank.

"It is possible to destroy these things, but there are consequences to doing so."

Underground battlefield

Throughout history, secret underground passageways have been used in many parts of the world to evade authorities and enemy forces. In Gaza, Hamas built up a network of tunnels that has helped it smuggle goods in from Egypt and launch offensive operations against Israel.

This time, it's relying on the concrete-enforced tunnels under Gaza to fight Israel's militaryas the government vows to demolish Hamas.

Cohen, who spoke to CBCNews by phone, was part of a team that authored a Rand reportpublished in 2017 thatlooked at Israel's operations against Hamas spanning the three-week Gaza War that began in late 2008, and the 51-day offensive in 2014.

While Israel had been well aware of Hamas'stunnels and one of its soldiers was taken hostage by militants who used a tunnel in 2005 the reportconcluded that the Israel Defence Forces (IDF) struggled to detect and dismantle the increasingly advanced subterranean network.

In 2013, the IDF found a tunnel near the Ein HaShloshakibbutz in southern Israel. The passageway was 1.8 metres high, and the entrance to itwas 22 metres below ground.

Israel estimated that the tunnel, which had been equipped with electricity and a large stockpile of supplies, was made of 450 tonnes of concrete and cost $10million US to build.

Lessons from 2014

Israel says that by 2014, Hamas was employing as many as 900 people to dig tunnels, which would each take on average threemonths to build and cost $100,000.

"It's their Manhattan Project, so to speak," said Harel Chorev, a senior researcher at the Moshe Dayan Center for Middle Eastern and African Studies at Tel Aviv University.

"They invested everything and anything that came into the Gaza Strip."

Chorev, who spoke to CBCNews by Zoom, said the entranceways to the tunnels in Gaza are frequently contained in buildings like houses, and shafts are built so weapons can be fired from underground.

During July and August 2014, Hamas used the tunnels to infiltrate Israel on four different occasions, killing 12 soldiers. According to the IDF, during that summer it discovered 32 tunnels, some as deep as 25 metres.

But after a ceasefire was called, there were reports and questions about how Israel's military should have been better prepared to confront Hamas's tunnels.

WATCH | Ambushes, booby traps andexplosivesawait in Hamas-controlled tunnels

How Hamas uses its labyrinth of tunnels in Gaza

12 months ago

Duration 4:03

Hamas uses an elaborate network of tunnels for everything from storage to staging attacks, and they pose a big challenge for Israel. CBCs Ellen Mauro breaks down how the tunnels change the war and make things more dangerous for the Israeli military and the Palestinians who live above them.

The Randreport, which included information from IDF soldiers, found that while Israel was experimenting with ground-penetrating radarand audio sensors to try todetect the tunnels, it didn't always work and they were frequently detected by intelligence or patrols who had stumbled upon them.

"We didn't internalize how vast [the tunnels]can be," Chorev said.

"The prospects now are far better than what it used to be in 2014, although I must emphasize, it won't be a walk in the park."

He says tunnel warfare is complicated enough, and in this case there could be hostages underground.

More training, new technology

Since 2014, there have been reports that Israel has experimented with half a dozen pieces of technology to better root out the tunnels, and all elite units have been given training to fight in the dark, closed-in environment.

In December 2015, the U.S. Congress earmarked an additional $40 million US in military aidspecifically for anti-tunnel technologies.

In the expected offensive, experts say Israel will likely use drones and robots.

"[Israel] had nineyears to learn from their struggles with the tunnels to 2014," said Shashank Joshi, the defence editor atthe Economist. He added that Hamas also had time to prepare and likely learned more tunnelling techniques from Hezbollah, a militant group from Lebanon.

Israel has been experimenting with technology, he said, including tracking phones and looking atwhere the signal suddenly drops, which could indicate that someone has gone underground.

But the challenges are enormous, and Joshiquestions just how accurate Israel's understanding of the tunnel labyrinth is, given that the military was caught off guard by the Oct. 7 attacks.

"It seems unlikely to me that Israel has a great picture of these things."