On Wednesday, a select group of Israel-based foreign reporters was taken to a section of the battlefield, which journalists described as “the fringes of Gaza City”. Nearly every building was destroyed or heavily damaged by aerial bombardment, artillery fire or advancing tanks and infantry.

Videos show Merkava tanks grouped in an encampment surrounded by tall sandy berms, almost certainly constructed by the armoured combat bulldozers routinely deployed with advance units. The defensive sand walls are likely to deny Hamas fighters the opportunity for hit-and-run attacks.

To an analyst, the position and posture of that 401st Brigade company show more than the Israelis probably wanted to. It tells us the advance will be slow, street by street rather than block by block.

It also proves that Gaza City’s hardest battle, the underground one, has not begun in earnest. Some tunnels may have been identified and destroyed as troops advanced, but that is likely a tiny part.

The 34 Israeli soldiers whom Israel has admitted have been killed so far were apparently killed individually or in small groups – when tunnel war begins, the numbers are likely to jump in bigger groups.

To enter the tunnels, Israeli forces will have to resort to military practices decades old and long forgotten to get around the challenges of fighting underground.

Identifying entrances

To gain a position to fight in the tunnels, Israel has to identify as many entrances as possible. For a system believed to be up to 500km (310 miles) long, those probably number in the tens of thousands.

Most are hidden, inside residential buildings, garages, industrial facilities, warehouses, under rubbish dumps and, after more than a month of bombardment, under heaps of rubble.

But Israel has been preparing to tackle the tunnels since the 2014 incursion into Gaza. Incessant surveillance by drones, using sophisticated software that analyses movement patterns and can recognise individual faces and match them to a database of known Hamas members, revealed hundreds or thousands of entrances.

Informants probably added more, and I would not be surprised if the Weasels (Samur) specialised Israeli tunnel-warfare unit, knows half the tunnel access points.

Mapping the tunnels

Knowing the entrances is useful, but even if all known ones were attacked, that would not make the tunnels unusable for Hamas. Most tunnels have several entrances at each end so some would always remain open.

The tunnel builders, Hamas, have a huge advantage as they know the network. Israeli software might offer hints connecting patterns of movement to reveal that two points are probably connected, but it does not reveal the underground routes, directions, or junctions.

To map the tunnels with whatever degree of accuracy, commandos must get inside, facing huge dangers and difficulties. The first is technical: Down there, GPS positioning devices are useless as satellite signals cannot penetrate the soil.

The solution will most probably use devices that combine magnetic sensors, not affected by going underground, and movement sensors like those used in step counters. A crude and imprecise system, but better than nothing.

Getting around

Once inside, the Weasels will most likely operate with night-vision goggles rather than give away their position using lights. They will not be able to use radios to communicate with units on the surface, so they will have to use field telephones, technology from over 100 years ago.

Soldiers will unroll wires, connecting them on the move, further slowing the advance. Even if they do not meet Hamas resistance, they must stop at every junction and assess where the branches lead.

A small force will have to be left at every side tunnel to defend from counterattacks. Every time they find a vertical shaft, which are almost always used for entrances, they will have to pause, map the position and relay it back to units on the surface.

Surface units will have to find the opening and secure it; if it is in territory not controlled by the Israeli army, they will either have to take it or tell the tunnellers to stop or go around it. This will repeat hundreds of times. In the past, Samur released videos of its tunnel-capable robots that might be useful as trailblazers, reconnoitring passageways and sending back night-vision videos. But they can be used on one level only, as they cannot climb ladders or obstacles.

Surviving inside

For practical purposes, everything has been analysed so far assuming that there is no opposition in the tunnels. That is completely unrealistic: Hamas has surely prepared to put up fierce resistance.

Most tunnels are probably booby-trapped with pre-positioned improvised explosive devices (IEDs). Those can be wired to remote detonators, but they can also be triggered by specialised detonators that react to light, vibration, noise, movement, and even increased carbon dioxide concentration when people are present.

The tunnels are laced with wires and cables that carry electricity, internet, telephones and military lines. Hamas may have observation and detection devices that would let them know where the Israelis are so they can remotely explode charges in that exact spot.

Israelis cannot simply cut all the wires because, like in movies, some detonators might be triggered when their electrical supply is severed. As everyone with a connection to mining knows, explosions in confined tunnels are far deadlier than on the surface. They spread further and suck out oxygen so those who survive the initial blast often suffocate.

Hamas may also ignite incendiary compounds that deprive occupants of oxygen and spread as high-speed flash fires or create thick, often toxic smoke. This would keep the tunnels mostly undamaged, allowing the Palestinian fighters to use them after they force the enemy out.

The Weasels will almost certainly have breathing apparatuses but wearing cumbersome masks and air tanks makes communication and combat more difficult.

Forcing Hamas out

Every commander, on both sides, prefers to avoid fighting in tunnels. Hamas probably cannot prevent Israelis from entering some tunnels but can try to deny them the freedom to operate in them.

Israeli command knows that its advantage in technology and weapons is considerably higher on the ground than under it, so it would prefer to flush Hamas out and fight on the surface. To do so, it may use chemical agents such as teargas, a little of which goes a long way in tight tunnels.

It is likely Hamas does not have enough protective gear for its tunnel fighters, so any gas-based agent could be effective. Even though Israel does not feel constrained by international conventions, as it does not regard Hamas as a legal combatant, I do not think it would use lethal gasses. That would cause additional international accusations that would be hard to deny.

Water has often been used in the past to flood tunnels and force their occupants out but there is simply not enough water in Gaza. But there may be other options. Egypt is said to have poured sewage into smuggling tunnels from Gaza.

Fighting

Urban fighting is difficult, requiring specific knowledge and equipment; tunnel combat is even more challenging and specialised. As military tunnellers found years ago, ordinary weapons are too big and cumbersome to use in confined spaces.

American Tunnel Rats in Vietnam often only used pistols but found that, when they fired, the flash destroyed their night sight for a long time. When using night vision goggles the problem is even worse, so it is likely that Israelis will carry smaller calibre weapons with sound suppressors, not so much to decrease noise as to prevent muzzle flashes.

Whatever firearms they choose, tunnel fighters will have limited firepower as only two can fire at a time, one kneeling, the other standing over them, blocking the field of fire for the rest of the team.

Hand and rifle grenades are almost certainly out, as well as any kind of rocket launchers. Stun and flash grenades might give the Weasels an advantage by rendering the enemy temporarily deaf and blind, but it is questionable if those can be used without danger to their own side.

In line with centuries-old practice, they will certainly be equipped with combat knives or machetes, as hand-to-hand fighting is certain to happen. There has been much talk about Israeli tunnelling attack dogs, but a military and police canine expert I talked to dismisses the idea.

Dogs are far too unpredictable in conditions of extreme combat stress and there were many cases when under flashes and noise of a firefight they turned against their own side, he explained.

Destroying tunnels

Hamas needs the tunnels and might only want to block some of them tactically but not destroy them altogether using small explosions to prevent the enemy from using a particular tunnel.

Digging under combat conditions is impractical and makes the diggers vulnerable the moment the obstacle is removed, so a blocked tunnel is likely to remain so for the duration of conflict. Israeli combat engineers had announced that they were testing a “sponge bomb”, a device containing two chemical substances that create rapidly expanding foam.

The idea is to instantly create a concrete-hard plug to block the tunnels, but there were mishaps in use, and it is not certain if the sponge bomb is ready for deployment. Rather than just block it, Israel wants to destroy every tunnel it takes, so it will have to make sure that the entire structures are caved in, not just the entrances.

In most cases, this cannot be done by simply placing explosives inside tunnels. For more permanent demolition, it is usually necessary to dig deep holes in tunnel walls and ceilings, fill them with blasting dynamite and detonate so that deep structure is shaken and soil caves in to fill it.

It looks quite implausible to embark on such a massive engineering undertaking during fighting, so Israel might see its task as first destroying Hamas fighters and then demolishing their entire underground network.

To get to the latter part might take Israel months and it must win the underground war first, something that will also take time.