"We built Israel," says Abbas, a young migrant worker from Salem. A decade ago, he began travelling illegally from the northern West Bank to Tel Aviv to work in construction. "We have no jobs, so the only option is to work in Israel."

Years ago, between 1948 and 1967, Palestinians sneaked across borders to work in their former fields in Israel. Those borders were erased after the 1967 Arab-Israeli war, when Israel occupied the West Bank, Gaza, the Golan Heights and Sinai. After 1967, Palestinians both from Gaza and the West Bank began working on Israeli construction sites. Now, with the building of the separation wall in the West Bank, those borders exist again.

Although men over 35 years of age can obtain security clearance to enter Israel for work, the younger generation have no choice but to travel via the paths their fathers and grandfathers used to walk legally. They are, in effect, the ghost workers of the Israeli economy.

Israel began erecting the wall in 2002. It has since slithered deep into Palestinian land, and its checkpoints and restrictions have crippled the Palestinian economy. All the while, ghost workers - those who cross the border illegally - continue to be the bedrock of Israel's economy. Indeed, its central bureau of statistics says about half of the approximately 220,000 foreign workers in Israel are illegal, while the Palestinian Workers' Union estimates there are between 35,000 and 40,000 illegal workers in Israel.

Deir Al-Hatab is a village near the settlement of Elon Moreh in the northern West Bank. It was once the scene of violent clashes between Palestinians and Israeli settlers. The villagers who fought the occupation now grow olives, but everywhere there is a sense of aftermath. Many of the villagers were sent to jail, many were militants, and today, many are blacklisted or forbidden from working in Israel.

At a small house in the village, a mother looks with pride more than grief at the posters of her son that cover a corner of the family's small living room. In the picture, Jalal, her son, stands in front of a mosque, the word shaheed(martyr) printed beneath his name. Tall, skinny, with high cheekbones, Jalal died in 2006, at age 26. He was shot by the Israeli army when the driver of the taxi he was travelling in tried to race away from soldiers at Hawara checkpoint.

"The driver took the settlement road to save time," Umm Jalal remembers. "When he saw that the soldiers had stopped two vans [full of] illegal workers he tried to escape; the boys asked him to stop, but he didn't." Jalal was killed outright and two other workers were wounded. There were mild protests from human rights organisations, but nothing has changed. "People will not stop going to work in Israel," says Ramzy Ouda, Jalal's brother.

They will not stop because Palestinians employed by Israeli contractors to build settlements earn three times more than those working for equivalent Palestinian employers, yet for Israelis this is still considered cheap labour.

In Wadi Fukin, eight kilometres from Bethlehem, the night is calm. The village is sandwiched between the Green Line and the separation wall. The village was destroyed in 1948 during fighting between the Israelis and Jordanians, which ended with a UN-backed ceasefire that became the Green Line on contemporary maps.

Wadi Fukin was the only Arab village permitted to be rebuilt after the 1967 war. The lights from settlements glitter across the valley, surrounding it on three sides.

Crawling with border police and monitored by Apache helicopters, thorny mountains stretch from the other side, providing a 35km secret route for illegal workers. Each morning, hundreds of workers cram into a narrow, fenced walkway, waiting under the wall's floodlights for Israeli border police to slowly let them in. The impatient ones squeeze through holes in the outer fence.

Before the wall went up, no permits were required and Palestinians travelled in relative freedom to work in Israel.

On the other side, Jerusalem is just starting to wake. It is 4am; the men - there are 21,600 Palestinians with legal permits according to official sources - need to be at work by 7am. At the end of their shift, they are required to leave at 3pm. Sleeping over in Israel is illegal, although some do.

Geha junction in Tel Aviv is busy in the mornings and afternoons, when Palestinians travel to and from work. Several minibuses queue and wait. The route is a gold mine for their young drivers. They are at the top of the underground hierarchy.

"You know how much I make in one month? Over US$4,000 [Dh14,692]!" Khalil, a Druze Arab Israeli, says smugly. Beside him several other young men wait. They shuffle workers around like vegetables, giving them bargain rides from the city to the checkpoints.

"Do you have a permit? Yes? Hop in!" says another driver, Khaled, 21.

The drivers charge illegal workers around $55 for the ride from the West Bank to Israel. Legal workers pay less.

"We do not argue with the drivers as they can report us to the police," says Abbas.

The hierarchy is based on where you were after 1948 and 1967. Palestinians whose families stayed in Israel after the 1948 war (the drivers are among them) were given Israeli papers and are called Arab Israelis by Israelis. Palestinians who came under Israeli administration when Israel occupied the West Bank in 1967 have only Palestinian papers.

Abbas says he and other workers from Salem travel first to Nablus, then to Ramallah and on to Bethlehem, where they cross the wall. Then they start their walk of up to four hours and arrive in Israel, where they are picked up by Arab Israeli drivers and taken to Geha junction.

Abbas makes about $800 a month, working five days a week, while squatting at the building site. He goes home at weekends via the same secret route.

A fake Arab Israeli ID costs between $400 and $800, according to Ibrahim, or Joseph as it says on his fake ID. One still uses secret routes to enter Israel, but you can get a job in the service industry, which pays double the rate for construction workers, says Morad, who works in a restaurant.

Those illegal documents allow people to exist within the skilfully woven Israeli security system - as long as the police do not run the ID through their database and find a mismatch between the photo on the screen and the person in front of them. For those who are apprehended, a prison term is as likely as deportation.

Both Ibrahim and Abbas, who are from the same village in the northern West Bank, now live undercover, enjoying the opportunities provided by their fake IDs.

But Israel is teeming with patrols and CCTV cameras are everywhere, so both young men know they cannot continue this double life indefinitely.

"You cannot live like this forever," says Morad. He will eventually return to settle down in his homeland, to open his own business after 15 years working in Israel.