Aconcagua was not always a mountain. For tens of millions of years it was a stratovolcano, erupting at the edge of a shallow sea where the Nazca Plate dives beneath South America. Then, roughly 8 to 10 million years ago, the angle of that dive flattened. The melting stopped, the pressure changed, and great thrust faults heaved the whole mass upward, lifting Aconcagua clean off its volcanic roots. The evidence is written in its flanks. Layers of ancient lava and ash sit alongside marine limestones, and the greenish, bluish, and gray bands visible near Puente del Inca are the floor of that vanished sea, raised now to dizzying heights. The mountain is a seabed in the sky.

The first people to reach the summit did not seek glory; they came for science, and for endurance. In 1897, a British expedition financed and led by Edward FitzGerald arrived to survey and to climb. Time after time, FitzGerald himself turned back, felled by nausea near 6,000 meters, watching the summit slip out of reach. Fearing the first ascent would never happen, he sent his Swiss guide ahead alone. On 14 January 1897, after roughly five attempts over six weeks, Matthias Zurbriggen stood on the highest point in the hemisphere by himself, the leader of the expedition still below him on the slope. A month later two more of FitzGerald's men made the top. The route Zurbriggen pioneered is essentially the normal route climbers still follow today.

Long before any European saw it, the Inca held Aconcagua sacred and climbed astonishingly high upon it. In 1985, near 5,167 meters, searchers found the body of a child, laid carefully on grass, cloth, and feathers within stone walls, surrounded by small figures and coca leaves. The clothing marked the child as belonging to the highest rank of Inca society. This was a capacocha, a ritual offering in which a young person was carried to a mountain shrine and given to the gods. The discovery is among the highest archaeological sites in the world. It is also a sobering reminder that this peak was a place of devotion and sacrifice centuries before it became a trophy, and that the child on its slopes was a real person, mourned and honored by people who believed they were entrusting one of their own to the divine.

Aconcagua remains as dangerous as it is alluring. Only about a third to 40 percent of those who try reach the top, and cold-weather injuries are routine. The dead include the experienced as well as the reckless: in January 2009, five climbers died, among them Italian climber Elena Senin, who perished shortly after summiting; her grieving family later donated an emergency shelter that now bears her name at high Camp Colera. The route up is studded with camps, from the village of Puente del Inca at 2,740 meters through the sprawling Plaza de Mulas base camp at 4,370 meters, said to be among the largest on Earth after Everest's, to the wind-scoured high camps near 6,000 meters. People come from all over the world, most of them foreigners, drawn by the chance to stand higher than anyone else in the Americas. The mountain asks only that they not mistake its gentle slope for mercy.