The war between Israel and Hamas has seen weaponry of the most modern kind, lighting up the skies and leveling the landscape.
But one ancient tactic in Gaza, a labyrinthian system of tunnels, is a key component of Hamas' military strategy. An Israel military spokesman told reporters that its Air Force is targeting these tunnels.
Under all the destruction in the Gaza Strip, there are miles of tunnels, some reportedly more than 100 feet deep. These tunnels were built and rebuilt over the years at a cost of tens of millions of dollars. The system is sometimes referred to as the Gaza Metro.
"They were constructed since even before 2007 when Hamas took control of Gaza, and they used to run from the Egyptian border into Gaza," said Gerard Filitti, senior counsel at The Lawfare Project. "They were initially smuggling routes to get weapons and other material to the terrorists in Gaza."
The tunnels are made of concrete and stone. They were first built in the early 1980s to smuggle in and move around goods like food and medicine after an Israeli blockade, according to experts.
"I would say it's, it's important not just for Hamas, but even for the population," said Eric Lob, professor of political science at Florida International University. "In some ways, it's important to survive if they're not getting adequately what they need, because Israel and Egypt are in control of the movement of people and even goods that come in and out of there."
Video posted by the Israel Defense Forces shows what it said are weapons hidden inside the tunnels.
"Over the years, they have expanded as a way for the terrorist organization Hamas to maintain a stockpile of weapons, including rockets hidden from view from spy satellites," Filitti said.
A 2014 report by National Geographic from inside Gaza said Hamas also taxes goods transported through the tunnels, which cost hundreds of millions of dollars to construct.
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Military experts said while valuable for transportation, storage and hiding, the tunnels also have a psychological use. The tunnels are an unseen, unpredictable angle designed to keep enemies wondering when and where they'll be ambushed.
During the Vietnam War, America became familiar with tunnel warfare as its enemies built miles of underground passages to spring up and attack U.S. soldiers.
"You need training for urban warfare and tunnel warfare. As the United States learned in Vietnam, it's different than fighting on an open battlefield," Filitti said. "So this poses a particular risk for the Israeli Defense Forces going into those tunnels physically to secure them, especially when you don't know the layout, you don't know what to expect down there."
On the social media platform Telegram, Hamas spokesman Abu Ubaida says some of the hostages kidnapped from Israel are being held in the tunnels.
Experts say the underground spaces could also be a challenge in a ground war in Gaza.
"We can't send in U.S. Special Forces or Israeli Special Forces to secure the release of our prisoners if we don't know where they are, if they're in miles worth of tunnel underground," Filitti said.
Video from an Israeli weapons maker shows how so-called bunker-buster missiles are built to burrow into the ground before exploding.
Pro-Israel voices, like Filitti, said Hamas using these underground passages to store weapons puts Gaza citizens living near them at risk of being injured or killed in the event of an attack on the tunnels by Israel.
"When you're hidden underground and your stockpile of weapons is underground, you are preserving your ability, not just to commit terrorist atrocities against Israel, but against your own citizens," Filitti said.
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