Last week I traveled to the south of Israel to report on the rocket attacks by Hamas and discovered tools of war not in the sky but underground.

The Israeli media calls them variously attack tunnels or offensive tunnels. They are not new to warfare and are properly analyzed as expedient field fortifications. One hundred years ago on the Western Front they were called trenches. In the Negev Desert in 2014, the tunnels have given Hamas militants a significant advantage, because they are genuinely terrifying.

On Monday, Israeli Prime Minister Benjamin Netanyahu declared that military operations would continue until all

such “tunnels of terror” were destroyed.

The current Israel Defense Forces (IDF) combat operations against Hamas are chiefly driven by the discovery of a surprising cross-border tunnel that enabled a Hamas attack on July 17. Thirteen armed men were spotted emerging from a tunnel several hundred yards inside Israel, near Kibbutz Shiva, a farming community. The IDF struck with high explosives, and the attackers fled. Analysis showed that the attackers were aiming for the more than 100 farmers asleep and unaware — perhaps for abduction, perhaps for murder.

Since the Shiva incident, the IDF has concentrated on discovering and destroying as many attack tunnels as possible. More than 30 have been eliminated so far. The tunnel entrances in Gaza are always concealed inside buildings. The IDF detonates the entrances and exits and seeks to destroy the whole tunnel by drilling from the surface and inserting high explosives. It is slow, perilous, inexact work, and much of the ground fighting between the off-and-on cease-fires is devoted to tunnel destruction.

On July 23, I visited a farming community in the Eshkol region, south of Gaza, which is close to where the IDF thwarted the tunnel attack six days before and has seen four dead and several more wounded from mortars in thepast 24 hours. There I sat with a widow, Anna (not her real name, to protect her identity in an active combat zone), to learn what it is like to be the target of a terrorist attack. Anna told me that all the children of the kibbutz were sent away weeks ago and that about 100 adults have stayed to maintain the community.

The kibbutz houses are so close to the Gaza line — less than a mile — that the Israel’s Iron Dome interceptor system of anti-missile batteries does not protect them. On the Red Alert app that Israelis use for rocket attacks, users often see “Eshkol region.” When the siren sounds, Anna has about 15 seconds to enter a secure room built to withstand a direct hit by a Qassam rocket, which is usually loaded with ball bearings or other metal pieces for shrapnel.

Although the rockets are frightening, the attack tunnels present Anna with a whole new threat. The authorities do not want her to lock the door of her secure room because they could not reach her if she became unconscious. This means that a Hamas raid would leave her defenseless in the secure room, a target for murder or abduction.

“My front door,” she said, “she can kick it in,” pointing to her 13-year-old deeply loyal terrier.

Anna said that some weeks ago, the farmers believed they heard sounds underground and complained to the authorities. Searchers could find no indication of tunnels, but no one was reassured. The attack tunnels discovered elsewhere so far make the southern reaches of Gaza like an anthill. The excavations are 40 to 90 feet deep, tall and wide enough for a man with a weapon to pass up to a mile underground.

The IDF reports finding weapons, high explosives and other ordnance in niches along the tunnel walls. The IDF also discovered Hamas militants carrying zip ties and narcotics to help in abductions.

It may be that the rocket barrage that started in mid-July brought the IDF to the region prematurely for Hamas’ purposes. Israeli security sources assert that Hamas was still preparing what is called the Rosh Hashanah terrorist tunnel plot, which would send 200 militants into Israel not only to murder civilians but also perhaps to capture parts of Israel and hold off the IDF with hostages.

Hamas’ rocket attacks — stretching from the Negev communities near Gaza deep into the country to Tel Aviv’s Ben Gurion International Airport, which was temporarily closed to air traffic — represent victories for its campaign to date. Although 90 percent of the rockets have been intercepted by the Iron Dome system, the random attacks have worn down morale and resources.

I visited a household in the city of Sderot, within sight of the Gaza line, that was hit by a rocket that passed through the Iron Dome. There I found a giant, ugly hole in a cinder block outer wall and Israeli citizens feeling drained, frightened and cursed by fate. The rocket killed no one, because they all dashed sleepily for shelter, but the kitchen was wrecked.

Despite such damage and fright, the attack tunnels represent a far darker twist in the conflict that wears down Israelis’ collective sense of well-being and security. To hear scratching in the night and worry that Hamas attackers are coming out of the ground works on the imagination far in excess of the facts. The possibility of the attack tunnels is the weapon that cannot be countered with armor, infantry, fighter jets or drones.

The IDF says that it is in a race to destroy as many attack tunnels as possible before this round of fighting ends. The IDF acknowledges that as soon as the combat units pull back, the tunnel diggers will begin anew. Like the trench warfare of a century ago, expedient field fortifications such as tunnels and bunkers are most difficult to defeat and can be thwarted only at very high risk to the offensive combatants. Just like in World War I, one more “big push” to defeat an entrenched enemy will always frustrate and fail. The attack tunnels of Gaza look to be the nearest Hamas has come yet to defeating the vastly competent and better-equipped IDF.