A Hamas tunnel network hundreds of miles long and up to 260 feet deep, like this one filmed by Reuters back in 2014.
Hamas drove a blindfold crew to a secret location and gave them a tour.
One expert describes it as the "Viet Cong times ten."
Freed hostage Yocheved Lifshitz called it "a spider's web."
"We started walking in the tunnels where the earth was moist, and it was always humid. We reached a hall where we gathered, some 25 people."
This video released by Israel's army purports to show part of the tunnel network.
Troops will have to enter and battle Hamas militants, while trying to avoid killing any hostages, snatched during the October 7 Hamas offensive in which Israel says some 1,400 people, most of them civilians, were slaughtered.
Sources familiar with the matter say the tunnels have many purposes: attacks, smuggling, storage, operational and transport.
The Gaza strip is just 25 miles long but security analysts say the network of tunnels beneath it is extensive.
Israel has spent millions of dollars on tunnel detection and destruction, including a sensor-equipped underground barrier that it calls an "iron wall."
Israel blockades Gaza, so the tunnels have enabled Hamas - which released this video in 2014 - to bring in weapons, equipment, money and people mainly from Egypt.
They're one reason the militant group is stronger in Gaza than in the occupied West Bank, where Israel's settlements, military bases and monitoring devices make it harder to smuggle from Jordan.
Israeli security sources say the past weeks' deadly bombardments have caused little damage to the network.
Hamas naval commandos launched this seaborne attack targeting Ashkelon, for example.
A former deputy commander of the Israeli army division that tackles the tunnels, Amir Avivi, called it a "whole city" deep below Gaza, connected to at least a thousand rocket-launching positions, also underground.
"They feel safe inside the tunnels, they can emerge from the tunnels out and attack our forces anywhere. This is why in all the operations we saw previously, even operations that went as long as 51 days, with the air force attacking again and again and again, the ability to really inflict serious damage is problematic."
Israel enjoys overwhelming military superiority.
But Hamas will seek to undermine that by forcing soldiers to move underground in cramped spaces the militants know well.
Hamas would also try to kidnap soldiers, according to a former head in Israel's Mossad spy agency.
A security source from one of Israel's neighbors told Reuters that Hamas' tunnels from Egypt remain active, facilitated by Egyptian military officers - though it wasn't clear if their bosses knew about it.
U.S. Defense Secretary Lloyd Austin has said what likely awaits the Israeli troops is, quote, "a lot of IEDs, a lot of booby traps, and just a really grinding activity."
Maybe worse, he said, than Iraq's nine-month battle to retake Mosul from Islamic State.
