South on the Ruta 40
From the Valle de Uco, the route picks up the Ruta Nacional 40 just north of Pareditas, the small junction settlement that serves as the obligatory transit point between Mendoza and San Rafael. There is fuel at Pareditas if needed before the long run south.
South of Pareditas, the Ruta 40 enters one of its most compelling stretches. The section between here and Malargüe, paved in its current form only since 2019, replaced a detour through San Rafael and shortened the journey by 100 kilometres. The road runs through a transition zone — the valley's irrigation giving way to dry plains, the mountains growing larger and more abrupt to the west, and the vegetation thinning to scrub. This is the northern edge of the Payunia volcanic field, and the evidence accumulates as the kilometres pass: dark basalt lava flows, extinct cones on the horizon, and the occasional anomalous black formation interrupting the pale desert soil.
For much of the drive south from roughly halfway between Pareditas and Malargüe, white plastic cylinders become a recurring feature in the pampa on either side of the road — spaced out in a regular grid extending to the horizon in every direction. These are the surface detectors of the Pierre Auger Observatory, the largest cosmic ray detector in the world, which covers 3,000 square kilometres of Pampa Amarilla. The array consists of 1,600 water-filled Cherenkov tanks, each equipped with electronics, solar panels, and communications antennae, spread in a triangular grid with 1.5 kilometres between each unit. At the legal speed limit, it takes roughly an hour to pass through the full extent of the array — a sense of scale that is itself a kind of exhibit. The observatory, a collaboration of 17 countries and around 350 scientists, studies ultra-high-energy cosmic rays: particles arriving from deep space at velocities approaching the speed of light, carrying energies far beyond anything produced by accelerators. The atmospheric conditions of southern Mendoza — altitude between 1,200 and 1,400 metres, minimal light pollution, extreme dryness — were decisive in the site selection.
The town of Malargüe appears at the end of the drive without ceremony: a modest provincial centre on the Ruta 40, at an elevation of around 1,408 metres (4,620 feet), flanked by bare volcanic hills and with the Andes close to the west. Fuel at the YPF La Cordillera on the way into town; provisions from the Vea supermarket as needed.