Here's the bottom line when it comes to fighting an enemy hiding in a labyrinth of tunnels — you don't want to send your troops there.
"It's very dark and it's claustrophobic and it's narrow. And you lose your sense of direction immediately. You lose completely a sense of time," said Daphne Richemond-Barak, an Israeli military analyst and the author of Underground Warfare. The Israeli military has taken her into Hamas tunnels that crossed from Gaza into southern Israel.
"They are humid and suffocating. So when you walk in them, you would be disoriented for sure, but also incredibly afraid," she added.
Hamas claims to have 300 miles of tunnels in Gaza, a subterranean complex that effectively serves as an all-purpose military compound. According to Israel, the underground space includes military headquarters, sleeping quarters, as well as workshops to make and store rockets.
The Israeli military has overwhelming firepower compared to Hamas. But the group's underground passageways crisscross Gaza and are designed to allow Hamas fighters to quickly surface and strike Israeli troops without warning.
Building tunnels for two decades
Hamas has been shoveling for a long time in anticipation of such a battle.
Chuck Freilich remembers the tunnels were part of the Israeli security discussions when he was Israel's deputy national security advisor in the early 2000s. He sees the vast expansion of the Hamas tunnels as part of Israel's broader intelligence failure in underestimating Hamas.
"You don't believe that the enemy can do X and so you don't see it," he said. "Then in retrospect, it always turns out that there's lots of information in the system, but people, smart people, ignored it."
With Israeli ground troops now deep inside Gaza, the tunnels are becoming a key factor in the fighting.
The Israeli military said its ground forces were recently near the Erez crossing point, the northern border of Gaza, when troops spotted Hamas fighters emerging from a tunnel. The Israeli troops fired on them, killing or wounding the Hamas members, the military said.
And when Israeli fighter jets unleashed a powerful airstrike on the Jabalia refugee camp on Tuesday, targeting a Hamas commander, the Israelis said the attack collapsed tunnels used by the group. Palestinians said the strike caused one of the highest civilian casualty tolls to date.
Hostages assumed to be in the tunnels
Freilich said Israel faces another big challenge because Hamas is believed to be holding more than 200 hostages underground.
"I think there's all sorts of fancy technology being used to try and map out the tunnels," he said. "But we may not know everything there is. Some booby trap is awaiting you at every turn. And of course, the hostages are probably down there somewhere."
In a recent military briefing, Israel said Hamas has constructed military headquarters directly beneath Gaza's largest hospital, Al-Shifa.
"Hamas terrorists operate inside and under Shifa Hospital and other hospitals in Gaza with a network of terror tunnels," said Rear Adm. Daniel Hagari, Israel's military spokesman.
Hamas vehemently denies this. The group says Israel makes these accusations to justify a relentless bombing campaign that's already killed thousands of Palestinian civilians.
Hagari argues that it is Hamas that's cynically putting civilians at risk.
"Hamas not only endangers the lives of Israeli civilians, but also exploits innocent Gazan civilians as human shields," he said.
Hamas first used the tunnels to shock Israel in 2006. The group dug a passageway under the Gaza border fence and emerged on the Israeli side, kidnapping an Israeli soldier, Gilad Shalit. He was held for five years, and ultimately exchanged for more than 1,000 Palestinian prisoners.
When Israel upgraded its Gaza border fence a few years ago, it included a concrete barrier that went deep underground in an attempt to prevent Hamas from tunneling beneath it.
Israel's military has also formed a mobile unit to detect tunnels and created a special team trained to fight in the tunnels.
Still, Richemond-Barak says Israel is probably reluctant to send troops underground because the terrain would be so favorable for Hamas.
"Most military doctrines recommend against sending soldiers into the tunnels. Why? Because of the very high risk," said Richemond-Barak.
The U.S. learned the harsh reality of fighting in the tunnels when American troops battled the Viet Cong in Vietnam. In the decades since, militaries have developed new weapons to combat them, like bunker-busting bombs and high-pressured water hoses to flood and collapse the tunnels.
"What we're likely to see is Israel using a combination of these different methods and potentially even new ones," Richemond-Barak said.
She says Israel knows destroying tunnels is hard. Five years ago, Israel discovered that the militant Hezbollah group had dug from southern Lebanon into northern Israel.
She says it took the Israeli military six weeks to demolish five tunnels – just a tiny fraction of what Hamas has built beneath Gaza.
Greg Myre is an NPR national security correspondent who was based in Jerusalem from 2000-2007. Follow him @gregmyre1.
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