There's nothing new in underground strongholds: tunnels of Iwo Jima, Cu Chi tunnel network in Vietnam, Tora Bora caves in Afghanistan - to name just a few from recent history. And, of course, mines and industrial "tombs" of present day war in ukraine. The thing that makes Hamas tunnels unique, is their location under a dense urban area, which takes, the already difficult, urban warfare to another level.
Yes, there were (or still are) tunnels going into Egypt, that were used for years to smuggle weapons, munitions, etc. into the Strip. The Israelis claim they destroyed those tunnels in the first few days, but whether they were able to completely destroy them, just shut them down temporarily, or even locate all of them is open question.
They used to have many of those, to send out infiltration teams, launch raids against Israeli border forces, and kidnap soldiers (the famous Gilad Shalit case) in the past. But the Israelis recognized the threat and begun to search and destroy them and, eventually, they installed an underground border
wall to prevent any further incursions - at least in theory, but in practice there's nothing they can do to prevent them - Hamas can always dig another tunnel under the concrete barrier, and as long as they don't use any power tools they can avoid triggering the sensors. It's a dangerous job, and the capacity of those "hand made" tunnels is low, but some of them could still be there - according to some sources, assault tunnels have been used on October 7. However the remaining cross border tunnels are of little value at this stage, there's not much that a small team could do to the Israeli forces in the deep rear - any acts of sabotage against fuel and ammo dumps can be achieved using drones - and the entire Gaza envelope is a closed military zone so there's little to no chance for deep terror raids. On the other hand, tunnels inside the Gaza Strip that lead behind the Israeli strike forces, are of great value - they allow Hamas to strike at the IDF's rear units (that Merkava that was taken out at point blank range was left without any infantry cover in a "safe" zone on fire support duty).
They won't last that long. As impressive as those tunnels are, they are facing two major problems.
First is their quality. Those tunnels were built without the use of specialised engineering equipment, since the Israeli blockade prevented their transfer, Hamas could only use jackhammers, drills, shovels. Same goes for the quality of building materials - You think that they were allowed to import high grade concrete into Gaza ? And what's the condition of those tunnels after a month of heavy bombardment? How many tunnels collapsed, how many underground chambers were hit directly with bunker busters, how many shafts were burried under the rubble ?
The second is sustainability. How much food, water and fuel they have in storage ? What about munitions ? Diesel generators and spare parts for them, and what about exhaust fumes? With all those buildings above, razed to the ground by the Israelis, even access to breathable air is in question.
We burned the Japanese on Iwo Jima with napalm, used a combination of carpet bombings and an infantry combing operations against the Vietnamese but it failed. Soviets took Tora Bora by assault, we did it with heavy bombardment.
What will the Israelis do ?
For starters they need to locate and "cut" the main underground lines that connect the, besieged, northern part of the Strip with the south - because at this moment Hamas still has freedom to move resources and fighters from one end of the Strip to the other. When, or if, it's done, they will go to work on the northern undergound. How ?
- With the main supply tunnels sealed, they could simply besiege those Hamas fighters that are trapped in the north - but it's unlikely, time is not on the Israeli side, they lose western support with every atrocity they commit.
- Raze the entire city of Gaza to the ground, collapse as many tunnels as possible and burry Hamas alive under the rubble.
- Chemical warfare. Could be lethal nerve agents, could be non-lethal smoke.
- Flush the tunnels with water. Back in 73, during the Yom Kippur war, Egyptians used industrial grade water pumps to breach Israeli lines along the Suez Canal, so it's not as crazy as it sounds.
- Combing operation. Once the ground forces secure an area above ground, they will locate the main communication tunnels and collapse them to cut off the block from the rest of the network. Afterwards they will move below (preceded by grenades) to clear the isolated section. Then move on to another city block: secure the surface, isolate local tunnels from the network, clear them. Rinse and repeat, one city block after another.
- Any combination of the above.