Johnson Varny is back after four months away, spent somewhere in the no man's land between Mali and Algeria. He'd been caught by the police here, who loaded him and 80 others onto the back of a truck, carting them back to Tin Sawatin. They spent two days and 700 miserable kilometers (400 miles) traveling southwest through nothing but sand and rocks. When they reached their destination -- a small collection of tin huts -- the police simply unloaded their human cargo and left them to fend for themselves.

"They treated us like animals," says Varny, 32. "One loaf of bread a day and water from a trough. They would never drink that themselves."

Yet here Varny is, back in Tamanrasset, southern Algeria, in the middle of the Sahara. He's a bit gaunter, perhaps, than he was before struggling through the 700-kilometer journey back here. He started out together with another man in Tin Sawatin, but only Varny has arrived. He says he buried his friend 40 kilometers (25 miles) short of the city, after the other man died of a burst appendix. "He didn't make it," he says simply.

Few refugees know the desert city of Tamanrasset as well as Johnson Varny does. He arrived here the first time in 1995, when he was 16 and had fled war-torn Liberia three years before. He's lost count of how many times he's been here since, both willingly and unwillingly. In 2000, he says, he made it on as far as Oran, an Algerian city on the Mediterranean coast. But before he could find a boat to take him to Europe, police caught him and deported him.

A Way Station

Surrounded by nothing but sand and rocks, Tamanrasset, with its population of 100,000, has been a way station for emigrants from sub-Saharan Africa for two decades. At first a few dozen people passed through the city each year, then suddenly it was hundreds, and now tens of thousands. Are they refugees, migrant workers, immigrants? Definitions are fluid here. They stay for weeks or months, often even years.

But then they move on, because ultimately, they all have the same goal: Europe, the continent that draws them north like iron filings pulled toward a magnet. Estimates say only around 15 to 20 percent actually arrive there. But these refugees don't let anything stop them from trying, not barbed wire or border guards, not bandits or the reports of the many hundreds of people who drown each year trying to cross the sea. They want to escape the hardship back home, so they chase after the dreams that satellite TV has planted in their minds.

In these 20 years, Tamanrasset has undergone an astonishing degree of development. There are paved streets now, sidewalks and electricity, entire new neighborhoods and even a university. These things were built with revenue from oil and smuggling, but also with the labor and the millions in cash left here by the city's short-term guests.

Where Undocumented Immigrants Live

The last residential neighborhoods at the western edge of the city give way directly to the desert. Another kilometer further out, slightly uphill from the city, is where the undocumented immigrants live. They have a few blankets, which get rolled up and stored under rocks during the day. There's some firewood and a couple of cooking pots, water bottles and a great deal of garbage. Other than that, they have no possessions beyond their own miserable living conditions.

No governments or aid organizations provide help. Algeria doesn't want to support migrant workers. A few good-hearted nuns occasionally bring blankets, an Italian organization donates money for medications and two or three doctors at the hospital will treat patients for free. There's no aid beyond that.

Varny coughs. He has tuberculosis and needs medication to treat it, but how is he supposed to pay for that? He, too, is living out here by the rocks. His entire group sleeps out here. In the mornings, they slip into the city, where they carry stones, paint walls or bake bricks. Or they simply wait. Even when they find work, it doesn't always go well. "You work for an Arab, for days, for weeks, and in the end, instead of giving you the money, he calls the police," Varny says.

Unstoppable Onslaught

Once they've saved enough money for their onward journey, these refugees could choose to board a bus for a comfortable ride to Algiers. Instead, armed with a bit of bread, a few cans of food and a water canister, they climb aboard a truck in the back courtyard of a building, setting out at night to avoid patrols. Once outside the city limits, the driver leaves the paved road and trundles northward on dirt tracks.

The journey to Morocco costs 4,000 ($5,000), Varny says, or 6,000 including the crossing to Spain, and Nigerians have a firm hold on this line of business. Women have it easier, he says, though there's the not insignificant drawback that they have to work off their debts in Spanish brothels.

Hundreds of thousands of people come northward hoping to scale the battlements of Europe, the old continent that's intent on building its walls ever higher, even considering using drones, offshore sensors and satellite-driven search systems to fend off this onslaught. The EU plans to spend up to 2 billion on new security systems at its external borders.

But that won't stop the immigrants, who start their journeys on the Ghanaian coast, in the villages of Côte d'Ivoire or in capital cities such as Kinshasa. In many cases, an entire family pools its resources, or even an entire village. Often it's the first-born in a family who sets out, sensing an inner obligation to seek out adventure. More than anything, though, they want to escape the miserable conditions they're leaving behind.