What to know about Hamas' tunnel system beneath Gaza

• Further complicating any Israel ground operation, at least some of the hostages kidnapped in the Oct. 7 terrorist attack were being held in tunnels, per a Hamas spokesperson.
• The IDF announced Friday that it was "expanding" ground operations in Gaza in addition to a massive bombing campaign. Israel has called up 300,000 reservists and Prime Minister Benjamin Netanyahu has said he anticipates a "long and difficult war."

How Hamas uses the tunnel system

• Hamas uses the tunnels to smuggle goods and contraband, store weapons and supplies, and train and barrack fighters outside the view of Israel's advanced intelligence services and beyond the reach of its air force.
• Hamas also uses underground facilities to assemble and store parts of its large arsenal of rockets and launch platforms.
• In the event of an expanded ground operation, the tunnels will force Israeli soldiers to contend with the risk of ambushes and booby traps in unfamiliar terrain.

The size of the tunnel network

Above ground, Gaza is one of the most densely populated places on Earth, with 2 million people living in just 140 square miles.

• Egypt and Israel imposed an extensive blockade on the enclave after Hamas took control in 2007, making the subterranean network of smuggling and transport routes all the more important.
• It's unknown exactly how extensive the tunnels are, but experts say they've grown in scale and sophistication over the past two decades, with some being equipped with electricity, lighting and rail tracks.
• They likely span large parts of the Strip, reaching more than 100 feet beneath the surface in some places and ending at dozens of hidden access points. Hamas' leader in Gaza, Yahya Sinwar, claimed in 2021 that the militant group had around 310 miles of tunnels in Gaza.
• Officials with the Israel Defense Forces told the New York Times in January that new intelligence indicates that the tunnel network is collectively between 350 and 450 miles long with around 5,700 separate shafts leading down the the tunnels.
• The officials said the new estimates were based on documents and computers captured in the war. The Times could not independently verify the estimates, which varied between Israeli officials who had access to different intelligence.
• They claimed that one of newly uncovered tunnels was wide enough for a car to drive through it, while another extended for nearly three football fields and was hidden under a hospital.
• In the past, the militant group also excavated tunnels across the Israel-Gaza border to launch attacks on Israeli forces, such as in the 2014 war.

Israel's strategy to take out the tunnels

• Just as the tunnels have grown more sophisticated, so too has Israel's strategy to contain them.

• In addition to conducting numerous ground and aerial operations to collapse tunnels or seal up access points over the years, Israel built a sensor-equipped underground anti-tunnel barrier below a fence spanning its entire border with Gaza.
• During the 2014 war, Israel launched a ground offensive into Gaza to destroy parts of the tunnel system but faced challenges in detecting, fighting in and demolishing them, according to the RAND Corporation.
• Taking lessons from that war and its 2018 operation against Hezbollah, Israel has enhanced tunnel warfare training for its soldiers. It's also developed new technologies to detect and collapse tunnels as well as robotic platforms to map and fight in subterranean environments.
• Attempts by Israel to take out tunnels from the air, including with massive "bunker buster" bombs, have often resulted in civilian casualties because Hamas has intentionally built them under civilian infrastructure.

Where the tunnels came from

• Tunnels have been used in Gaza since at least the early 1980s, after the city of Rafah was divided by the new border recognized in the 1979 peace treaty between Egypt and Israel.
• With only one crossing point along the new reinforced border, families in Rafah were separated and the city's economy was severely fractured, thus prompting the construction of underground tunnels through which family members could communicate and smugglers could shuttle goods.
• The use of the tunnels by militants came to light during the first Palestinian Intifada, beginning in 1987.

Go deeper:

• What to know about Hezbollah's ties to Iran and threat to Israel
• What to know about the Gaza Strip and who controls it
• What to know about Hamas' military capabilities
• What to know about Israel's military strength
• How Israel's Iron Dome missile defense system works

Editor's note: This story was updated with new details reported in January on the extent of the tunnel network.