Bob Guerro is short, muscular and clean-shaven, a retired Marine who works for the Arizona Department of Corrections. His wife, Rayo, his high school sweetheart, works at a local daycare center. Their 22-year-old son and 18-year-old daughter both still live at home. In 2004, the Guerros sold their California home and bought one in Surprise for $260,000. They didn't know it, but they arrived at the tail end of a 50-year dream that was about to abruptly end.

That dream began in 1954, alongside what was then the modestly sized city of Phoenix, when a man named John Long created Maryvale, a master-planned community of wide streets, cul-de-sacs, nearly identical affordable homes and a big shopping mall. A few years later, the feds said that air conditioning could be included in home mortgages. Maryvale set a template for suburban growth in Phoenix and the nation, and air conditioning made it possible for that growth to occur without newcomers broiling in 120-degree summer heat.

Arizona began to shift from an economic past built on the "four c's" of citrus, cotton, copper and cattle into a future built on, well, growth. The change was rapid: Phoenix's population grew from just over 100,000 spread over about 17 square miles in 1950, to four times that a decade later. Today, 1.5 million people live in Phoenix proper -- which     covers more than 500 square miles -- and millions more live in the amoeba-shaped appendages that spread out for miles in every direction.

The pattern of this remarkable growth, which was mimicked by other Western cities, was apparent from the beginning. As early as 1959, a Phoenix planning task force worried about the city's tendency to "leapfrog" outward, leaving huge tracts of land vacant close to its center -- a trend that has just become more pronounced. In spite of efforts to encourage infill development, most homebuilding has happened farther and farther out on the fringes, gobbling up desert land at alarming rates. Surprise, for example, was just a little town, separate from greater Phoenix, in 1990. Now it's been swallowed by sprawl.

Though this tendency to grow away from the city center, its jobs and its amenities seems counterintuitive, it makes sense from an economic and even a cultural viewpoint. Western cities typically have plenty of private land onto which to grow. If they run out, they develop ways to convert public land -- Las Vegas has grown onto once-federal land and Phoenix and Salt Lake City co-opt state land (see related story, page 14) -- much as early settlers homesteaded and staked mining claims on public land. Many of Phoenix's newcomers -- especially between 1970 and 2000 -- were retirees, so there was less need to build housing near employment. Most importantly, though, the farther you go from the city, the cheaper land tends to be.

And that's the only way to keep these exurban growth machines running. People are not flocking to Arizona's urban fringe to grow oranges or build computers; they're coming because it's cheap. In a 2004 survey by the Behavior Research Center of Arizona, 86 percent of respondents listed affordable housing as "somewhat" to "very important" factors in their decision to move to the Phoenix area. Just 67 percent rated jobs as important, and only 59 percent cited the weather.