- Hamas has used tunnels to stage attacks in Israel and Gaza, the Israeli military says
- The military has destroyed some tunnels, but says it believes there are many more
- Hamas used a tunnel to capture an Israeli soldier in 2006
- A tunnel into Israel discovered last year showed increasing sophistication
Israel is confronting a problem beyond the Hamas rockets screeching overhead -- a threat underfoot.
The Israeli military says it is trying to demolish a sophisticated network of tunnels that run through parts of northeast Gaza, under the border and into southern Israel.
Hamas has already used the tunnels several times in the past few days to attempt assaults on Israeli soil.
The first attack, on July 17, was foiled but prompted Israel to announce a ground incursion into Gaza with the stated aim of taking out the tunnels.
Another assault through tunnels s few days later resulted in clashes that killed more than 10 Hamas fighters and four Israeli soldiers.
The assault near the town of Sderot appeared to target two communal areas "where farmers are trying to conduct their daily lives," said Israeli government spokesman Mark Regev. The Hamas fighters were disguised as Israeli soldiers, according to the Israel Defense Forces.
The clashes forced area roads to close, residents to shelter in their homes and tied up security forces for hours.
The method of attack, in which militants spring out unexpectedly from underground, has struck fear into Israelis living near Gaza.
"Your enemy is about to blast his way into your dining room from below the floor while you are feeding your family. Sounds like a B-rated horror movie, right? This scenario is one real example of a Hamas tunnel discovered just in time by the IDF leading into a kibbutz communal dining hall," Benay Browne Katz, a volunteer medic and grandmother who lives in Jaffa, told CNN.
The tunnel network has also been used during combat inside Gaza, the Israeli military says, allowing Hamas fighters to pop up and fire on soldiers or toss grenades before dropping back out of sight.
Israeli military officials refer to the underground works as "Lower Gaza" and suggest at least some of the war is being waged underground.
The tunnels aren't a new phenomenon. Hamas used one in 2006 to capture the Israeli soldier Gilad Shalit and take him back into Gaza. He was held captive for five years until a deal was struck for his release in exchange for more than 1,000 Palestinian prisoners.
Memories of his capture were revived by a foiled attack over the weekend, in which one Hamas fighter who entered Israel through a tunnel was found to be carrying tranquilizers and handcuffs, according to the Israeli military.
'A whole industry'
Israel received a warning of the growing scale and sophistication of the underground threat last year with the discovery of a tunnel that ran from the Khan Younis refugee camp in Gaza and emerged near the Israeli kibbutz of Ein Hashlosha.
Uncovered in October
, the tunnel was wired for electricity and communications. It was also high enough for a man to stand, walk or run through.
It was long, about 1.7 kilometers (roughly one mile), and deep, at least 18 meters (59 feet). Its interior was fully lined with an estimated 500 tons of concrete.
"We're talking about a whole industry, and not a small group that's organizing it," Maj. Gen Shlomo Turgeman, commander of the IDF Southern Command, said at the time.
Israel responded by halting supplies of concrete and construction materials into Gaza, tightening an already restrictive blockade.
But the current conflict suggests that tunnel building has continued at a steady clip. The Israeli military says that during its incursion into Gaza, it has so far found scores of different access shafts leading to about 30 tunnels.
Some of the entry points were underneath people's houses
, officials said, and Israeli military uniforms were stashed inside some of the tunnels.
The IDF said it has destroyed about half the known tunnels so far with controlled explosions or heavy earth-moving equipment. The substantial construction of the tunnels, some of which stretch for kilometers, has made them difficult to demolish, Israeli military officials said.
"We're taking action right now to neutralize those tunnels and we'll continue the action as long as necessary," Israeli Prime Minister Benjamin Netanyahu said last week.
The Israeli military says it believes there are "tens" of tunnels still to be found -- and some of the fiercest fighting it has encountered has been around the tunnel entrances, suggesting a strong desire by Hamas to hold onto them and protect them.
Hamas started off using tunnels to burrow under different Gaza border -- the one with Egypt. The aim of those tunnels wasn't to mount attacks, but to ferry goods. The underground routes into Egypt were a way to circumvent the Israeli blockade on Gaza.
The Gaza-Egypt tunnels "are used to bring in food, to bring in gas, livestock, anything else that the Gazans need. And also, according to Israel, are used to resupply Hamas. That is how they get their weapons into Gaza within this blockade," said CNN's Paula Hancocks, who visited some of the tunnels in 2009.
Despite efforts by both Israel and Egypt to crack down on the smuggling tunnels, they've proved hard to stamp out.
"They've been bombing these tunnels between Gaza and Egypt for years and still they keep popping up," Hancocks said. "They're very, very basic, very dirty, but they are very quick to create."
Israel's current effort to destroy the tunnels into its own territory is taking a heavy toll -- on Gazan civilians and Israeli soldiers.
The question is whether the incursion will remove the threat for good.