Until this latest war, if you asked most Israelis about the threat from Gaza, they would probably start talking about Hamas rockets.
But that has changed over the past few days of fighting, for two reasons: One, the much-heralded success of the Iron Dome missile defence system has all but neutralised Hamas's rocket threat. Two, and far more troubling for Israelis, they have woken up to the true extent of the subterranean threat from Gaza - the tunnels that snake underneath the densely populated coastal territory into Israel proper.
What do the tunnels
The tunnels are hardly crude. With years of experience digging passageways under the Egypt-Gaza border to smuggle weapons, people and goods into the blockaded territory, Hamas knows how to burrow.
The tunnels discovered by the IDF are reinforced by concrete walls and ceilings. Some are 90-feet deep and extend more than a mile in length, terminating inside Israel not far from residential neighbourhoods. Israeli troops have discovered phone lines, electricity wires, pulley systems and stockpiles of explosives and weapons in the tunnels.
Many of the tunnels have multiple branches and a multitude of exit points, which explains why the precise number the IDF says it has found keeps fluctuating. As of Tuesday, the number was 66 access shafts as part of 23 tunnels.
They begin inside buildings in Gaza, where it is easy to conceal digging from outsiders. Their end-points inside Israel are difficult to detect because the terminus often is not dug out until Hamas fighters are ready to pop up and perpetrate an attack - heavily armed, usually well camouflaged and sometimes disguised as Israeli soldiers.
Why is this threat
Israel has yet to work out an effective way to systematically address the threats the tunnels present.
Hamas could use them to kidnap Israeli soldiers, as it did with Gilad Shalit in 2006, or even to kidnap civilians. Israeli troops have found Hamas infiltrators in recent days armed with tranquillisers and handcuffs for just such operations, according to the IDF.
For its part, Hamas has made clear that one of its main goals is to pull off a successful kidnapping. An abducted Israeli could be used to bargain for the release of Palestinians incarcerated in Israeli prisons.
Infiltrators also could use the tunnels to sneak behind enemy lines and perpetrate attacks inside Israeli cities, towns or kibbutzim.
Hamas also is using the tunnels to ambush IDF soldiers. Four Israeli soldiers were killed on Monday morning after an infiltration; two died Saturday during an earlier infiltration.
There have been at least five tunnel infiltration attacks.
How can Israel combat the tunnel threat?
There is no technological fix to the tunnel problem. Instead, Israel's primary method for combating the tunnels is decidedly low-tech.
Israeli ground troops are looking for tunnel openings in the buildings they are searching inside Gaza. Troops in Israel are on the lookout for new infiltration attempts..
What do the tunnels
mean for the duration
of the war?
Before Israel launched its ground invasion on July 17, the Israeli government seemed reluctant to send troops into Gaza. Israel quickly agreed to a ceasefire offer a week into the conflict (Hamas ignored it) and gave Hamas at least two other chances to change its mind.
But now that Israel has awakened to the true extent of the tunnel threat and Israeli troops are already fighting and dying in Gaza, Israeli Prime Minister Benjamin Netanyahu seems determined to have the IDF destroy as many as it can.
"The operation will be expanded until the goal is achieved: restoring quiet to the citizens of Israel for a long period," Netanyahu said on Monday.
If the war ends before the tunnel threat can be addressed adequately, the IDF's job in Gaza will have been left unfinished. Though Israelis are agonising over the death toll on their side, they do not want those soldiers to have died in vain. This is seen inside Israel as a war of necessity, not of choice.
How are ordinary Israelis reacting?
The mini-wars with Hamas in 2009 and 2012 were fought on Gaza's turf, not inside Israel. Both the Gaza conflicts and the Lebanon war involved deadly rocket fire into Israel, but there were no pitched battles on Israeli streets.
But now the existence of tunnels through which terrorists can infiltrate threatens to bring the war into Israel - a frightening thought for Israelis.
The country still remembers the Maalot massacre of 1974, when Palestinian terrorists slipped across the Lebanese border and took more than 100 children hostage at a school in northern Israel. Twenty-five Israelis were killed in the incident, which ended when Israeli troops stormed the school building.
This war has turned into a nightmare for many Israelis, particularly those burying loved ones. But there's a reason IDF troops are still in Gaza: They're working to avert something worse.