Marie and Francisco Caro needed a home after they married, but like many people in California's Central Valley, they didn't have enough money to sign a lease or take out a mortgage.

They were tired of sleeping on separate beds in crowded homeless shelters, so they found a slice of land alongside the Union Pacific Railroad tracks in downtown Fresno. The soil was sandy and dry, prone to rising up into clouds when the autumn winds came. All around, farm equipment factories and warehouses loomed out of the dust, their walls coarse and sun-bleached like desert mountainsides.

Even a strong person could wither in a place like this, but if they wanted to build a home, nobody was likely to stop them. So Marie and Francisco gathered scrap wood and took their chances. They raised their tarp roof high like a steeple, then walled off the world with office cubicle dividers. Thieves stayed outside and so did the wind, and the sound of the passing freight trains softened.

When I visited the Caros in January, a fire burned in an overturned oil barrel, warming the cool air, and fresh-cut Christmas tree boughs hung on the walls for decoration.

While Francisco chopped wood, Marie, 43, confided that she wants to live somewhere else.  All she needs is a modest place with a sink and a gas stove, she said, maybe even a little television for watching church services on Sundays.

But until times change, she said, she'll be happy in her self-made abode, cooking on top of the oil barrel, making meals with whatever food God brings.

"He gives us bread," said Marie, a Fresno native who quit school in the 10th grade, ashamed of a learning disability that got in the way of her reading. "I'm just waiting for my home."

From the well-kept interior of the Caros' place, one can hardly see the jagged rows of tents and shanties on the vacant land around them. About 200 people have built informal habitats along the railroad tracks, primarily poor whites and migrant workers from Mexico.

There are many names for this fledgling city, where Old Glory flies from improvised flagpoles and trash heaps rise and fall with the wavering population. To some it's Little Tijuana, but most people call it Taco Flat.

Just to the south, under a freeway overpass, there's another camp of roughly equal size called New Jack City where most of the residents are black. Even more dwellings are scattered throughout the neighborhood nearby, appended to the walls of industrial buildings and rising up the flanks of freeway spurs.

Fresno, which the Brookings Institution ranked in 2005 as the American city with the greatest concentration of poverty, is far from the only place where people are resorting to life in makeshift abodes. Similar encampments are proliferating throughout the West, everywhere from the industrial hub of Ontario, Calif., to the struggling casino district of Reno, Nev., and the upscale suburbs of Washington state.

In any other country, these threadbare villages would be called slums, but in the U.S., the preferred term is tent city, a label that implies that they are just a temporary phenomenon. Many journalists, eager to prove that the country is entering the next Great Depression, blame the emergence of these shantytowns on the economic downturn, calling them products of foreclosures and layoffs.

While there's some truth to this notion, the fact is that these roving, ramshackle neighborhoods were part of the American cityscape long before the stock market nosedived, and they are unlikely to disappear when prosperity returns. The recent decades of real estate speculation and tough-love social policies have cut thousands of people out of the mainstream markets for work and housing, and the existing network of homeless shelters is overburdened and outdated.

People such as the Caros are part of a vanguard that has been in crisis for years, building squatter settlements as a do-or-die alternative to the places that rejected them. This parallel nation, with a population now numbering in the thousands in Fresno alone, was born during the boom times, and it is bound to flourish as the economy falters.

"The chickens are coming home to roost," said Larry Haynes, the executive director of Mercy House, a homeless outreach organization based in Southern California. "What this speaks of is an absolute crisis of affordability and accessibility."