The helicopter came in like revelation itself, a gash of white light hitting the desert, and a terrible bellow of rotors. Heart pounding, waking straight out of sleep, I took a second to get my bearings. I was in my sleeping bag under a scrawny palo verde tree, not enough cover to hide me from a Phoenix police helicopter scouring the dry mountainside for criminals and indigents. I held tight and waited for the light to find me. Before it did, the helicopter suddenly withdrew, like a claw yanked back into the sky. I sat up and watched it veer across the incandescent grid of the city, a brilliant tapestry of light rolled out to every horizon.

I was not in the wilderness, not even on its edge. I was in the middle of Phoenix, camped illegally on Lookout Mountain, part of the Phoenix Mountain Preserve, hemmed on all sides by streaming freeways and traffic lights. I watched the helicopter as long as I could, until it was swallowed by the city.

The dark mountains of Phoenix have always attracted me. At night, they seem like a sanctuary. Suspended over the baritone rumble of the city, I found better sleep here, even with the helicopters, than in a motel on scratchy sheets. Beyond comfort, though, Lookout Mountain gave me a vantage on the city, a place where I could see an entire lay of urban ancestry.

In high school, I used to climb these mountains, my Friday nights spent bedazzled by the lights below. That was more than 20 years ago, and since then the population of greater Phoenix has more than doubled to nearly 4 million residents, its lights now saturating the sky, blotting out all but the brightest stars. The sight used to be awe-inspiring, and now it is almost terrifying.

Growing faster than any other population center in the nation, Phoenix is balanced on an environmental tightrope. Thirty new skyscrapers are proposed for downtown alone, while metastatic sprawl carves up surrounding desert. At the moment, there is a robust water supply — for greater Phoenix alone — but the city’s water-wealth has created a growing inequity in the state. Even while Phoenix has become Arizona’s solitary Green Zone, depending almost entirely on water imported from mountain reservoirs and the Colorado River, it is having its own problems, whole pieces of land subsiding into falling water tables. Phoenix seems either on the verge of unparalleled success or catastrophic failure. At this point, it might be hard to tell the difference between the two.

This is not the first time unchecked growth has filled the Valley of the Sun. If you lift the rug of Phoenix, buried directly below you will find the remains of an ancient city, a Neolithic version of Phoenix. The first communities appeared in the low basin of the Salt River 3,000 years ago, as shown by remains recently discovered under the new Phoenix Convention Center. From there, prehistoric settlements took an escalating course of empire, filling the basin to overflowing. They sprawled all the way south to Tucson, while satellite communities appeared even north of Flagstaff. They grew until they were no longer able to sustain themselves. Then, their civilization fell.

They are known as the Hohokam, a Pima word meaning “all used up.” The doubled first syllable accentuates the sentiment, telling of a people that completely burned itself out. When Anglo settlers arrived shortly after the American Civil War, they found a desert studded with grand adobe ruins, vestiges of an inexplicable culture. They called their new settlement Phoenix, imagining themselves rising from the ashes of a lost city.

Sitting up in my sleeping bag, I looked across Phoenix and it seemed resplendent, unassailable. My eye moved along seamless corridors of light where the once lonely towns of Mesa, Tempe, Chandler and Goodyear converged, absorbed into a greater municipality. Curiously, these lights define the same settlement patterns archaeologists have found among Hohokam communities. The two cities overlap as if made for each other.

The original Anglo settlers began digging canals in the mid-1800s, and wherever they dug they unearthed the remains of Hohokam canals below. For the next 150 years, engineers planned new canals throughout this growing city, and nearly each one follows an original Hohokam grid, as if pouring water back into an ancient hydraulic empire, bringing a ghost to life. New and old waterways follow the same gradients, the same courses. The Hohokam built nearly a thousand miles of canals and laterals, quenching fields of cotton, corn, pumpkins, amaranth and beans. They filled basins for domestic water at innumerable villages and town sites. Modern Phoenix followed suit, becoming a cotton-growing mecca, a city of water in the heart of the desert. Communities sprang up in the same places that Hohokam settlements once stood. Even the saline content of modern Phoenix’s water matches the chemical signature of Hohokam water just before the fall. The two cities lie along a similar environmental trajectory, and they may face a similar end.

When archaeologists study the Hohokam, they see a civilization that finally collapsed under the weight of drought, overpopulation, and ensuing social disarray. When they look at Phoenix, they see potential for the same.

Tom Wright, a local contract archaeologist, told me, “The Hohokam kept extending their canal systems farther and farther from the river. You tend to find their later sites under present-day Chandler or out in Goodyear. They kept increasing from the valley outward. The irrigation canals were the system that fed growth. Water was the lifeblood that flowed through these communities.

“Now the lifeblood is traffic. The city grows along patterns of freeways. Water doesn’t seem to count any more. We take it for granted. But whether people know it or not, like the Hohokam, we are an irrigation society living on imported water, and we’re outstripping our resources. Sound familiar?”

Wright is not a melancholy doomsayer. Like many of his colleagues in the city, he is merely a connoisseur of the way civilizations rise and fall. He knows the signs to watch for.