Around the turn of the 20th century, when sulfurous water was discovered bubbling out of the ground, cattle ranches and homesteads began to proliferate across the valley. One of the first deep water wells was drilled around 1915, when Texas farmers began adopting the oil industry’s turbine pump. Overnight, this innovation allowed agriculture to stray deep into arid climates, and in the span of a generation, the valley became home to a thriving agricultural economy. In the late 1990s, during the first few years of what would eventually turn out to be a 19-year-and-counting Arizona drought, only about 15,000 acre-feet of water were estimated to have percolated into the aquifer each year, while 100,000 were being pumped out; as the valley continued to warm throughout the 2000s and 2010s, with rainfall and snowmelt plummeting, estimates for recharge went unrecorded, as annual pumping soared to 200,000 acre feet. Once, it had been possible for ranchers to develop natural springs into watering holes using only a shovel. Now, after watching water levels drop 100 to 300 feet in 35 years, some farmers wondered how long they could go on.

Until the last three decades, the technology to make detailed maps of these underground waterways did not readily exist. It wasn’t until 2015, in fact, that NASA published its first comprehensive study of global groundwater reserves. The mission began in 2002, with the launch of the Gravity Recovery and Climate Experiment (Grace), two satellites that follow each other in orbit, measuring changes in gravitational pull. The mission’s primary purpose was to look at ice-sheet depletion, but over the next several years Dr. Jay Famiglietti, the senior water scientist at NASA’s Jet Propulsion Laboratory, and his team noticed that many of the most significant sites of water loss were actually below ground. Of the planet’s 37 major aquifer systems, they discovered, 21 were on the verge of collapse. In the Great Plains, farmers had exhausted a third of Ogallala’s potable water in just 30 years. In California, the Central Valley aquifer was showing signs that it could drop beyond human reach by the middle of this century. But the worst declines were in Asia and the Middle East, where some of the planet’s oldest aquifers were already running out of water. “While we are so busy worrying about the water that we can see,” Famiglietti told me, “the water that we can’t see, the groundwater, is quietly disappearing.”

In the United States, it is disappearing most rapidly in the rural agricultural belt extending from Kansas to California. Without ready access to more traditional stores of water, many farmers have been forced to rely even more heavily on groundwater, pitting them against local residents watching their wells go dry. In 2014, in Tulare County, Calif., 7,000 people ran out of drinking water. The next year, wells hit a record low, as 64 percent recorded declines nationwide and one in 30 failed in Western states. Squeezed by drought and tightening regulations, large farms started to seek out lesser-known pockets of cheap water. In rural Arizona, where there are essentially no groundwater regulations governing irrigation, they found an ideal destination. “What the smart money is doing is looking around and saying, ‘Where else can we go where there is no regulation?’ ” Robert Glennon, a professor of water law and policy at the University of Arizona and the author of “Water Follies,” told NPR in an interview. “And that is Arizona.”

Arizona was particularly attractive to Middle Eastern farmers. A policy of unregulated pumping on the Arabian Peninsula had, in 40 years, drained aquifers that had taken 20,000 years to form, leaving thousands of acres fallow and forcing Saudi Arabia and others to outsource much of their agricultural production. In 2014, a Saudi Arabian-owned company, the Almarai Corporation, bought 10,000 acres in the town of Vicksburg, northwest of Sulphur Springs Valley, planting alfalfa to ship halfway around the world to feed Saudi cattle. Then, a United Arab Emirates farming corporation, Al Dahra, bought several thousand-acre farms along both sides of the Arizona-California border. These purchases were perfectly legal, but many residents felt these newcomers were essentially “exporting water.” At least once, the Sheriff’s Department in Vicksburg deployed five deputies to stand guard at a town-hall meeting.

With less rain and snow reaching the desert floor, overpumping has rendered a semi-renewable resource finite, touching off the kind of resource war perhaps more familiar to coal camps and oil boomtowns. Hydrogeologists use the phrase “groundwater mining” to describe situations in which the rate of water withdrawal exceeds the rate of replenishment. For some, the metaphor offers a stark lesson. “If we know we’re mining the water, let’s just say it,” said Richard Searle, when I visited at his ranch outside Willcox. At 63, Searle still cuts a frontiersman’s profile; a cutting-horse competitor and former bank manager, he is descended from a prominent ranching family and formerly served as county supervisor. Part of the reason groundwater mining in the valley hadn’t forced a reckoning earlier, he said, was that water was ubiquitous to the point of being invisible. Local farmers were never required to put meters on their wells, he pointed out, which meant that nobody knew exactly how much water was being pumped, much less how much was left. “Long term, people say we should search for a solution,” he said, “but they don’t want to be the ones to suffer.”

Seated at his desk, Searle reached and opened a glass cabinet, lifting out arrowheads and a stone ax blade that he dug out of his ranch over the last 50 years. “You know, we weren’t the first ones here in this valley, and we weren’t the first ones struggling with water,” he said. His face turned pensive, and he spoke for a time about the ancient Hohokam and Tohono O’odham tribes, which traversed this part of the Sonoran Desert for thousands of years without digging deep wells. “But the mining industry isn’t a long-term industry,” he continued. “Name me a long-term mining community. Ajo, Pearce — those are ghost towns. Pecos was like this: a natural resource mined until the town fell apart around it. If we die, it’ll be a slow one. If the whole county dries up, it’ll be just a blip on the radar.”

When the corporate incursion to the valley began in earnest, in 2003 or so, local farmers had been mining the aquifer gently for the last 60 years. Even as the amount of irrigated acres more than doubled, from around 40,000 to 100,000, the potential consequences for the valley’s water supply weren’t yet apparent to them. “I could see acres being planted,” said Alan Seitz, who farmed chile peppers and alfalfa for close to 40 years. “It just happened over a period of time.” Plain-spoken and self-effacing, with a gray mustache and Stetson, Seitz advises local farmers on pest control, operating his business out of a Ford F-250, which was littered with fertilizer studies and geological maps. He spent most days on the road, covering hundreds of miles as he checked fields. It wasn’t uncommon to see farmers shifting growing patterns or fallowing acres; new drilling rigs or freshly tilled acreage didn’t excite much chatter. What drew Seitz’s interest, in 2010 or so, was the depth to which the farmers were drilling. “Those of us that had been in the valley growing corn, cotton, alfalfa, historically, we couldn’t drill deep wells,” he said. The cost made it prohibitive. When he saw people drilling down to 1,000 feet or 2,000 feet, Seitz knew straight away that moneyed operations intended to plant nut trees.