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."

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.