Shortly after midnight on Tuesday, Israeli forces announced that they had blown up a tunnel that allowed Hamas to “enter Israel by sea.”
The sea tunnel was unusual, a sign that Hamas has developed deadly new methods to attack Israel. The armed group has miles of tunnels under the Gaza Strip – one US official likened them to “miniature cities” – but the exit to that tunnel was on a beach.
Among the possible reasons why Israel delayed sending troops to Gaza after the Hamas attack on October 7, one stands out, according to military experts: the tunnels.
Beneath the tiny coastal strip and its more than 2 million people lies a vast network of underground paths, rooms, cells and even roads for vehicles. Hamas, which oversees Gaza, is believed to be hiding weapons, fighters and even command centers in the underground chambers.
According to the Israeli military, Israeli warplanes attacked 150 underground targets in the northern Gaza Strip on Saturday night.
For Israeli forces involved in any large-scale ground invasion, one of the biggest challenges will be the tunnels, which Hamas has worked for years to improve. For the people living above the tunnels, one of the most frightening questions will be how to survive the underground war.
“We should have no illusions about how this will turn out,” said Gen. Joseph L. Votel, the former leader of the United States Central Command, which is responsible for the Middle East. “There will be bloody, brutal fighting.”
The tunnel network will be one of the biggest challenges for Israeli forces engaged in a ground invasion. Credit…Sergey Ponomarev for The New York Times
In an interview, he recalled the final days of the battle for the Iraqi city of Mosul, where Islamic State fighters hid in a series of tunnels in 2017. “Our Iraqi soldiers used bulldozers to clear out ISIS fighters who were literally dug in the rubble,” he said. “It was very, very brutal.”
Tunnels have been a part of life in Gaza for years, but they proliferated after 2007, when Hamas took control of the enclave and Israel tightened its blockade. The Palestinians responded by building hundreds of tunnels to smuggle in food, goods, people and weapons.
According to the Israeli military, the tunnels cost Hamas about $3 million each. Some are made of precast concrete and iron and have medical rooms for treating wounded fighters. Others have rooms 130 feet underground where people can hide for months.
In Israel, the tunnel system is often referred to as the “lower Gaza Strip” or “subway.”
Yocheved Lifshitz, an 85-year-old woman held hostage by Hamas for 17 days this month, described being marched for miles through a “spider’s web” of tunnels. She told reporters on Tuesday that Hamas fighters led her through the wet and damp underground corridors to “a large hall where about 25 abductees were concentrated.”
After two or three hours, they moved five people from their kibbutz to a separate room, she said.
At a news conference on Friday, Rear Adm. Daniel Hagari, an Israeli military spokesman, accused Hamas of building tunnels and other facilities under Gaza’s Al-Shifa Hospital, the territory’s largest medical center. He played a monitored audio recording and showed an illustration of the underground complex.
General Votel, who visited a tunnel controlled by the Lebanese Hezbollah militia near the Israeli border, said he was “amazed by the effort involved in building these things.”
“It wasn’t just holes in the ground, it was architecture,” he said. “They were connected to rooms and built to withstand impacts on the surface.”
As Hamas expanded the underground system, it hid the entrances to the tunnels in houses and other small buildings on the Egyptian side of the border, said Joel Roskin, a geology professor at Bar-Ilan University in Israel during his time in Israel military busy with tunnels. These tunnels made it possible to smuggle goods out of Egypt.
The tunnel system extends to the Israeli border in the north.
A decade ago, Egypt made an attempt to destroy the tunnels along its border by pouring sewage into some of them and leveling houses that obscured entrances, Mr. Roskin said.
Israel has limited insight into tunneling activities on the Egyptian side of the border, he added. Many of the networks end in northern Sinai, but the Egyptian government has rarely allowed Israeli researchers or government officials to visit the area, so it is not clear how many cross-border tunnels remain.
A Palestinian clears a tunnel after Egyptian authorities pumped wastewater into the tunnel system in 2013.Credit: Ali Ali/European Pressphoto Agency
On October 14, Hamas released a video showing a group of militants emerging from tunnels and carrying out a mock attack on Israeli tanks. To music that could have been part of a “Call of Duty” soundtrack, the fighters then drag fake Israeli prisoners away from the tanks and throw them headfirst into the tunnels before slipping back into the underground passages themselves.
“This is what awaits you when you enter Gaza,” the video says at the end.
Daphne Richemond-Barak, a tunnel warfare expert at Reichman University in Israel, said she doubted anyone knew how many kilometers of tunnels Hamas had. Some analysts put the number in the hundreds. The group’s leader in Gaza, Yahya Sinwar, said in 2021 that there were 310 miles of tunnels in Gaza.
In 2018, Israeli forces destroyed a tunnel more than a mile long.
Some of the tunnels were built with mechanized digging equipment, but the tunnels that Hamas is believed to be using as a launching pad for attacks on Israel are being dug by hand or with shovels to avoid detection, officials said. The sandy terrain makes digging the tunnels easier.
Typically, the tunnels through which Hamas fighters move are about two meters high and one meter wide, experts said. The narrow width can be a nightmare for soldiers who have to move through in single file.
Palestinians assess the damage to their homes after an Israeli airstrike in Khan Younis, Gaza, on Thursday. Most tunnels lie beneath densely populated areas. Photo credit: Yousef Masoud for The New York Times
“From a defense perspective, it is an operational challenge for the IDF,” Ms. Richemond-Barak said.
Soldiers and officers who have worked to clear tunnels in Gaza in the past say the military typically refrains from sending people there. “Hamas has prepared its tunnels,” Ms. Richemond-Barak said. “You’re probably booby trapped.”
There is no “probable” about the booby traps, said Col. Amir Olo, the former commander of the elite Yahalom combat engineering unit, which is responsible for dismantling tunnels. Colonel Olo was part of an Israeli operation called “Protective Edge” in 2014 with the stated aim of destroying Gaza’s tunnel system during a two-week ground invasion.
Booby traps — typically bombs that are either triggered remotely or explode when something crosses a tripwire — are ubiquitous, he said. In 2013, six Israeli soldiers were injured and one blinded when a booby trap exploded as they tried to push a camera into a Hamas tunnel.
Soldiers who have cleared tunnels say entering a tunnel is the last thing they want to do. “By using the tunnels, the enemy can surround us from the rear and attack us,” Col. Olo said in an interview.
Ben Milch, an Israeli-American who cleared tunnels with the Israeli military during the 2014 Gaza war, said his unit came under repeated fire while destroying about 13 tunnels.
Mr. Milch said he and other soldiers were initially unsure where to look for entrances, which were often in densely populated areas near mosques and homes. But then the troops discovered telltale signs like pulleys next to buildings.
Airstrikes and remote sensors can destroy tunnels, but at some point Israel will have to send people if it wants assurance that a network has been completely dismantled, military officials said.
“Ground forces are needed to counter tunneling tactics,” said a RAND Corporation report on the 2014 Gaza war. “Even after the conclusion of Protective Edge, the IDF faced real technological challenges in detecting, engaging, and ultimately attacking tunnels destroyed.”
An Israeli military unit called Samur, or Wiesel, specializes in underground warfare and trains in mock tunnels in Israel.
The entrance to a tunnel discovered by Israeli forces near Kissufim, Israel in 2017. The Israeli military regularly trains infantry on how to destroy tunnels. Photo credit: Uriel Sinai for The New York Times
An Israeli reservist in the West Bank said other infantry units were also being trained in tunnel warfare.
He described a technique called “purple hair” that could be used to locate the tentacles of a tunnel. Israeli troops throw smoke grenades into a tunnel and then watch as purple smoke emanates from houses in the area. The smoke, the soldier said, signaled that a house was connected to the tunnel network and needed to be sealed off before soldiers descended into the tunnels. The smoke moves through the tunnel system like strands of hair, he said.
But the sea tunnels represent a dangerous future trend, said Ms. Richemond-Barak.
In 2018, Israel destroyed one that reached meters into the sea, perhaps the first of its kind to be discovered. Hamas divers could have used the tunnel to enter Israeli waters undetected.
After the Israeli military announced on Tuesday that it had destroyed the tunnel to the sea, it released video of another incident. Officials said it showed Israeli forces bombing Hamas divers who had emerged from a tunnel along Gaza’s coast and were attempting to enter Israel near Zikim Beach.
“Hamas continues to innovate in the area of underground warfare and utilize its knowledge and expertise in new, novel ways,” said Ms. Richemond-Barak.
Jeffrey Gettleman and Gal Koplewitz contributed reporting from Jerusalem.