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

If as anticipated, the Israel Defence Forces push into Gaza as part of a ground offensive in retaliation for the deadly attack on Israel by Hamas, they will be fighting in a densely populated strip that is home to more than two million 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 weapons while bunkers deep underground house Hamas's command centres.

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

Hamas has said it could be holding some of the 200 hostages seized in the Oct. 7 attacks, including Israeli soldiers, underground. The Israeli government says more than 1,400 people were killed in the attacks.

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

Israeli forces tasked with clearing them will be vulnerable to ambush and booby traps while the Palestinians living above them are at great risk of being caught up in a war that has already killed more than 4,000 people 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," said Raphael Cohen, a senior political scientist in the Washington, D.C., office of the Rand 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 military as the government vows to demolish Hamas.

Cohen, who spoke to CBC News by phone, was part of a team that authored a Rand report published in 2017 that looked 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's tunnels — and one of its soldiers was taken hostage by militants who used a tunnel in 2005 — the report concluded 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 HaShlosha kibbutz in southern Israel. The passageway was 1.8 metres high, and the entrance to it was 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 $10 million 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 three months 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 CBC News 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.

The RAND report, which included information from IDF soldiers, found that while Israel was experimenting with ground-penetrating radar and audio sensors to try to detect 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 aid specifically for anti-tunnel technologies.

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

"[Israel] had nine years to learn from their struggles with the tunnels to 2014," said Shashank Joshi, the defence editor at the 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 at where the signal suddenly drops, which could indicate that someone has gone underground.

But the challenges are enormous, and Joshi questions 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."