A photo taken in March of an entrance to a tunnel near Kibbutz Nir Oz that was discovered by the Israel Defense Forces (Ruth Eglash)

JERUSALEM — Israel has said that the goal of the military ground incursion it launched into the Gaza Strip on Thursday night is to wipe out the tunnel infrastructure, built by Hamas and other militant groups, that it says snakes under the border from the strip into Israel.

In a statement announcing the ground operation, Israeli Prime Minister Benjamin Netanyahu said he had ordered the army "to inflict as much damage as it can on Hamas’ infrastructure, particularly its system of tunnels, one of which was used Thursday morning in an attempted infiltration into Israel."

He was referring to what the Israeli military described as a "foiled terror attack" on Kibbutz Sufa, an agricultural community that sits just over a mile from the Gaza border. The infiltration attempt, which the military said occurred early Thursday morning, involved some 13 militants exiting a tunnel less than a mile from the kibbutz. After an exchange of fire with Israeli soldiers stationed nearby and an aerial strike, Israel said, all of the militants were killed.

On Friday, as military analysts and experts in Israel began to digest what is being called the "next phase" of Israel’s operation in the Gaza Strip, journalist Yoram Cohen, Palestinian Affairs reporter for Israel’s Channel 1, described the coastal enclave as a place divided into two — a world above ground and a world below ground. The army estimates there may be more than a dozen tunnels in operation, although no one has given an exact figure.

At an army briefing in the first few days of the now 11-day-long conflict, Israeli military spokesman Lt. Col. Peter Lerner said: “Tunnel terrorism has developed in Gaza over the last few years.” He said that the tactic was first used widely during the 2008-09 war between Israel and Hamas in Gaza. However, there is evidence that it began earlier than that when Israeli soldier Gilad Shalit was kidnapped in a cross-border raid near Kerem Shalom, at the southern end of the strip, in 2006. Apparently, his Hamas captors used a tunnel to infiltrate into Israel.

Since then, the Israeli army has uncovered at least four such cross-border passageways and suspect there are many more. While some of the tunnels are quite crude, others reflect sophisticated engineering techniques, reinforced with concrete and even fitted with electricity and phone lines. According to reports, some of the tunnels are similar in design to those that once linked Gaza to Egypt and were mostly used for smuggling in goods. Over the past year, since President Abdel Fatah el-Sissi took power in Egypt, those tunnels have mostly been destroyed.

One such tunnel, which the army discovered a little more than a year ago, ends on the outskirts of Kibbutz Nir Oz, on the eastern side of the Gaza Strip. The army found food, a ladder and other personal items inside the passageway, which was supported by timber beams. Officials said they suspected that before it was blown up, it ran at least a mile from the Gaza town of Khan Younis into Israel.

According to Israeli military intelligence, the militants would ideally try to move at least 10 or more fighters through the tunnels and have them infiltrate an Israeli border kibbutz or military base quickly. One senior Israeli military official who spoke on the condition of anonymity said those infiltrations could lead to mass murder or hostage-taking.

“We know this is what they are planning, and that is why we get jitters whenever we find these tunnels,” said Lerner, the military spokesman.

Adele Raemer is an Israeli resident of Kibbutz Nirim, a little more than a mile from the Gaza border. Her community has been a target of persistent rocket fire since the recent flare-up in violence between Hamas and Israel started, but, she said Friday, she is much more frightened by the prospect of Islamist militants suddenly appearing in the heart of her kibbutz.

“The true terror is the fear of these tunnels,” said Raemer, an English teacher. “We cannot live this way. We cannot live with the fear that a tunnel will pop up in our community and people will jump out and start shooting or take prisoners.”

“As much as I feel empathy and sympathy for people living on the other side of the border and as much as I believe there is no military solution and that there really has to be a political solution, I am glad they went in," she said. "They did the right thing. They have to get rid of tunnels.”