For a quarter of a century, successive United States administrations have invested billions of dollars into a technological safeguard known as the "virtual wall." Stretching across thousands of miles of the arid, unforgiving terrain of the American southwest, this sprawling network of high-tech surveillance towers, thermal-imaging cameras, radar systems, and, more recently, artificial intelligence was pitched as a modern solution to an age-old challenge. Proponents promised that these digital sentinels would revolutionize border security, enhance the detection and apprehension of unauthorized border crossers, and ultimately save human lives by allowing authorities to locate individuals in distress swiftly....