with more than 2 million residents stretching

for 25 miles between Israel and Egypt.

But just underneath this sprawling landscape,

another layer exists, a vast labyrinth of tunnels.

The underground maze was largely built by Hamas.

The group designated by the US as a terrorist organization

controls the Palestinian enclave.

As Israel prepares for a potential ground defensive

in the territory following a broad assault

by Hamas on October 7th,

the subterranean network could play a key role.

So here's why experts say these tunnels could make

urban combats costly for both Israeli soldiers

and Palestinian civilians.

The underground network has existed for decades inside Gaza.

Previously it was largely used as a roots to smuggle goods

and weapons from Egypt into the narrowest strip of land

that's twice the size of Washington DC.

But since then, the tunnels have evolved

to become a key military asset for Hamas.

- I went in 2014 with the IDF into a Hamas built tunnel

during the conflict at that time,

and it was a pretty crazy experience in a sense.

These were pretty sophisticated tunnels.

I'm about six foot, and they were, you know,

I didn't have to bend down at all in them.

They were very well reinforced with concrete.

There was communications lines running down the side

of them, and then they also had like, almost like a sort

of little rails or like a sort of railway line running

along the length of the tunnel.

- [Narrator] During Israel's seven week war on Gaza in 2014,

the Israel defense forces

or IDF said it destroyed 32 tunnels.

(bomb exploding)

They were used to hide weapons, equipments and troops,

but also allowed Hamas militants

to launch surprise attacks

using cross-border underground networks.

In the years since then, Hamas has continued

to develop its tunnel infrastructure,

and Israel has continued to target it.

This map, based on a graphic shared by the IDF

represents networks of tunnels that Israel said it damaged.

- I think the expectation is that there is

still a significant subterranean network

that Israel's gonna encounter if

and when it goes in into Gaza this time.

- [Narrator] Former Israeli security officials say

Hamas has built hundreds of miles

of underground tunnels in the last year or more.

Israel readying for a possible ground invasion of Gaza

by carrying out airstrikes

targeting the subterranean network.

- I think that the Israeli, the strategy will be to try

to destroy as much of the tunnel network as possible

before they launch a ground invasion.

- [Narrator] However, the operation

to destroy them could be challenging for Israeli forces.

One of the biggest issues is locating them.

- Gaza is a dense urban environment

and being able to discern precisely

where a continuous tunnel,

which may in fact be deep underground is located

as opposed to possibly natural pockets

of slightly differing geology,

or a host of other aspects of the built up environment,

that's a really hard challenge.

- [Narrator] Even if a tunnel is located,

additional factors including its depth can mean

precision bombs can struggle to destroy it.

- It depends not only on knowing precisely

where the tunnel is beneath the surface,

but on having the explosive power to be able

to damage a tunnel network that's beneath

perhaps layers of concrete, possibly layers of metal,

whatever the built up environment entails inherently,

as well as whatever materials have gone into construction

of the tunnel itself.

And the net result is that tunnels are

relatively invulnerable in many cases to air attack.

- [Narrator] And let's say any Israeli operation

to destroy the Gaza tunnel system could be complicated

by the presence of hostages.

Hamas' armed wing said in the statements on Telegram

that some of them were being held

in the underground tunnels.

- If the aim is to rescue hostages,

they can't be detonating explosives

within the tunnel complex, which might kill everyone inside.

It's going to require ground forces,

specifically tunneling forces who know how

to operate in these tight spaces,

who have trained extensively

in how to minimize the risk of ambushes

and how to avoid triggering various booby traps.

- [Narrator] One 85-year-old hostage who was released

by Hamas said she had been inside.

- There are a huge, huge network

of tunnels underneath, it looks like a spider web.

- [Narrator] Entrances to the subterranean passageways can

also be hidden within residential infrastructure.

Experts say as a war continues any mission

to target the tunnels also brings risks to civilians.