They are the real resistance. They are the lung through which Gaza breathes. True, missiles must pass along their subterranean tracks, Qassam rockets, too, Kalashnikov ammunition, explosives. But by far the greatest burden of the tunnellers of Gaza is the very life-blood of this besieged little pseudo-Islamic statelet: fresh meat, oranges, chocolate, shirts, trousers, toys, cigarettes, wedding dresses, paper, entire motor-cars in four bits, car batteries, even plastic bottle tops. The tunnellers of Gaza are bombed by the Israelis, they die in their own collapsing tunnels – and now they face a new Egyptian wall, even the fear of drowning. Terrorists they may be to the Israelis – the promiscuous use of this word makes it fairly meaningless these days – but heroes they are to the Palestinians of Gaza. Rich ones, too, perhaps.

But right now, Abdul-Halim al-Mohsen is worried about the Egyptians. He sits by the spitting log fire near the shaft of his tunnel, turning his hands to the flames, breathing in the thick blue smoke, a vast white tent above him casting his fellow-tunnellers into Rembrandt-like shadow, half-faces, thick pullovers, bright flames amid the gloom, the generator purring in the corner.

"Of course I'm afraid of the Egyptian wall," al-Mohsen says. "They will pour water down. How can we defeat this? We may drown." He holds out the palms of his hands towards me in that familiar "what-can-we-do?" gesture of so many Palestinians – but he is speaking in a matter-of-fact voice. The tunnels beneath the Gaza-Egyptian frontier are a business, a professional's game, Israel's bombs a challenge rather than a problem. There's even a four-truck miniature railway down one of the shafts. Money makes the wheels go round.

True to their treaties with Israel and the Quartet (of Lord Blair of Kut al-Amara fame), the Egyptians announced last month that they will build a wall – walls being the currency of the Middle East these days, from Kabul and Baghdad to the West Bank – between the southern-most rubble of the Palestinian Gaza Strip and Egypt, in order to break through and close down the "terrorist" tunnels. Foreign NGOs in Gaza dismiss this as the usual Egyptian window-dressing to please the Israelis – which means to please the Americans – adding that the Egyptian wall will only descend 18 feet beneath the ground, falling far short of the tunnels' depth. Perhaps it is in the tunnellers' interest to be more pessimistic. Al-Mohsen seems genuinely troubled by the Egyptian initiative.

"If they flood our tunnel, our dangers increase," he says. "It takes an hour to get out of the tunnel if we are stooping or on our hands and knees. When the Israelis are bombing, we clamber through to the Egyptian end – the Israelis won't bomb the Egyptian side – but if the Egyptians stop us, we will be caught by the bombing if the tunnel collapses."

I wonder about this, especially when Abu Wadieh invites us to look at the cavernous vault which opens in the far corner of the tent. This is no dirt hole in the ground but a solidly built stone-and-brick vertical tunnel, almost 15 feet in width and 90 feet deep – so deep that I can scarcely see the tiny arms of the men far below me as they heap bags of fruit on to a big steel hook – and more than half a mile long. A hawser whisks the bags to the surface as the generator whines and the men at the rim of the tunnel give them a gentle push so that they swing back into their arms. These men know their job. All profess to be uninterested in politics, of course. No weapons pass through their tunnel. Oh no, indeed.

A truck has backed into the tent, a squad of men piling fruit and vegetables and furniture and bottles of Egyptian Coca-Cola on to the lorry. I ask al-Mohsen – he swears he will be a construction engineer if peace (a muffled gasp here) looms – for his inspiration. He's seen pictures of tunnels before and he saw a film long ago in which foreign prisoners – British – escaped from a German camp through a tunnel. Of course. The Great Escape! Richard Attenborough and James Garner and Steve McQueen and the truck on railway lines which ferries them out of their Stalag. It accounts for the professional quality of the tunnel – even for the underground railway line. Though I don't choose to remind al-Mohsen of what happened to Attenborough.

But this is no laughing business. NGOs estimate that Hamas skims 15 per cent of the profits off the tunnellers' turnover, giving that august institution – excoriated by Israel, the US and Europe ever since they had the temerity to win the 2006 Palestinian elections – a quiet $350m (£225m) income per annum.

So while the world blockades Gaza and condemns the 1.5 million souls here to penury and – in some cases – near-starvation, Hamas supplies itself with all the concrete, building materials, iron and weapons that its plentiful supplies of money can buy.

While the EU gutlessly allows Israel to deprive Palestinian civilians of cement to rebuild their homes after last year's bloodbath in Gaza – because Hamas might use the cement to build bunkers – Hamas itself has more than enough cement to build a city of bunkers or a fleet of mosques, not to mention the buildings it has erected opposite Israeli troops at Erez.

In other words, the tunnels keep Hamas in pocket and Gaza alive. The Palestinian poor, of course, have to be fed by the United Nations. The tunnels thus represent not just a series of blood vessels between Gaza and Egypt, but a massive international hypocrisy.

Abu Wadieh, who employs 35 men working in and above al-Mohsen's tunnel, stands beside the crackling fire, a kuffiah wound round his head like a builder's helmet, rubbing his hands in the cold wind that pours into the tent as the latest truck carries its riches off to Gaza City.

"I'm afraid the men will all leave if there's another war," he says. "But they are experts. They know what to do."