Again and again the Gaza Strip is described as a prison.

Israel controls access, and thanks to its long-running confrontation with groups like Hamas, it has made it near impossible for Gazans to come and go from the tiny strip of territory.

But there is a very unofficial and well-known way in and out - the tunnels.

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The network is dug deep into the soft sand and runs under the border with Egypt.

They are used to smuggle in everything from cigarettes to food to weapons.

I have come to know some of the men who dig Gaza's tunnels, and in so doing I have gone underground to explore their dark and dangerous world.

The most disconcerting thing about crawling on hands and knees through these tunnels is the steady drip, drip of soft soil that keeps falling on you.

You start to wonder just how soon it will be before the whole thing collapses.

Usually it does not, in fact, collapse - despite the fact that many of the tunnels do not have anything to support the roof.

There is usually plenty of air, thanks to vents that are painstakingly dug upwards with thick pipes.

'Just one more'

Israel regularly complains the Egyptians do not do enough to search out and destroy tunnels, but the tunnellers say the Egyptians have been known to pump poisonous gas into tunnels as they uncover them - with fatal results.

Last week one of the tunnels did give way - probably because the Egyptians pumped in water from their side and weakened it.

One man died, and had to be dragged out by his feet 24 hours later by a member of one of the several extended tribal families which work in the tunnels.

"Constructing them is the worst job in the world," says a man with calloused hands who calls himself Abu Mutassem.

I have met him several times over the past few years, including three times inside one of his tunnels.

He always says this tunnel will be his last one, he has got enough money now.

But in reality he keeps on going - digging just one more.

  All dead tunnellers locally are regarded as martyrs or Shaheeds, even if their sacrifice was not for Islam but for profit

The money is good, each group of four tunnellers was at one point in 2006 getting more than £100 ($200) for every Kalashnikov gun smuggled through from Egypt to Gaza.

Recently the flow of small arms and bullets through their tunnels has all but dried up - such is the glut of weaponry already inside Gaza. There is now more than enough for everyone who wants one to have his own semi-automatic and a sidearm with plenty of ammunition.

But there are also more secret, so-called "military" tunnels, which are controlled by Hamas's highly-disciplined Izzeidien el Qassam brigades. No-one knows what sort of weaponry goes through there.

The Israelis believe it includes high explosives and some of the latest rockets that can target Israel's nearest big city, Ashkelon.

They are looking into military solutions to halt tunnelling such as deep penetration bombs.

Egyptian spree

Before Israel pulled all its troops out of the Gaza Strip in 2005, the Israelis regularly found and set off explosions in the tunnels, but even they were only partially successful in restricting the flow of arms.

Locals say there are now literally hundreds of tunnels along the border strip.

The tunnel operators, known locally as "snake heads", believe they have little to fear from the Palestinian authorities in Gaza - provided their rulers get a cut of the proceeds.

The culture of tunnelling is so entrenched in the southern city of Rafah that the main men's barber shop is called the Shaheeds' Saloon - with pictures of dead diggers and tunnellers plastered on the walls and mirrors.

All dead tunnellers locally are regarded as martyrs - or Shaheeds - even if their sacrifice was not for Islam but for profit.

  We are dipping our bread in blood
Relative of killed Gazan tunnellers

The smuggling industry ground to a halt earlier this year when Hamas troops blasted holes in the land border between Egypt and the Gaza Strip, and hundreds of thousands of Palestinians stocked up on supplies and luxury goods in a massive shopping spree inside Egypt.

But now the Egyptians have sealed the border again.

Underground, business continues to boom.

Petrol smuggling

The tunnel bosses are making their fortunes again - especially in goods made scarce by the Israeli and Egyptian blockade - such as petrol.

Such is the booming nature of their business, there are now even phone lines from one end of the tunnel to the other these days, and many tunnels now have air conditioning, electricity, winches to carry people and goods up and down, even vacuum cleaners to help remove loose soil.

  EGYPT-GAZA BORDER 12km (7.4 miles) long Egyptian side patrolled by 750 soldiers under 2005 agreement with Israel Border crossing terminal south of town of Rafah PA control of terminal under EU supervision collapsed after Hamas takeover of Gaza in June 2007 Border closed almost continuously since

On the Palestinian side of the border, men on shiny motorcycles, bought during the brief opportunity in Egypt, roar up to the petrol sellers and buy one litre of petrol in cold-drink bottles to pour into their thirsty fuel tanks.

That petrol that has been smuggled underground and dragged through the tunnels in plastic jerry-cans.

Which is why Helmi Erbaya died last week. Fuel spilled out of the jerry cans and his two sons, both tunnellers, passed out.

Their father rushed through the tunnel trying to save Nabil and Arafat, got both of them out, but himself succumbed to the fumes. 

Last Tuesday afternoon, Nabil also died in hospital.

"We are dipping our bread in blood," was how a relative put it as people lined up to shake hands with the mourners in a three-day ceremony under tenting outside the family home.

The other son, Arafat, has made a full recovery and vows not to go into the tunnels again.

But his younger brother, aged 14, says he has already been introduced to the art of tunnelling, and is ready to follow in the family footsteps - even if those footsteps may lead to death.