The pass shows a different character depending on which side you climb. From Argentina, the approach is deceptively gentle - a long, patient incline that eases up toward the tunnel mouth at around 3,200 meters. From Chile, the mountain shows its teeth. The slope plunges far more steeply, and the road has to claw its way up through a dramatic series of switchbacks, hairpin after hairpin stacked against the cordillera. Drivers crossing from Santiago face one of the more vertiginous mountain ascents in the Americas before they reach the relative calm of the summit tunnel.

Opened in 1980, the Tunnel of Christ the Redeemer runs 3,080 meters straight through the rock, and the international border sits at its midpoint - the literal end of Chile Route 60 and the start of Argentina's Route 7. The tunnel was a transformation. Before it, the road crossed over the top of the pass, and the engineering it eliminated was staggering: the new bore lowered the maximum elevation by some 600 meters, did away with 65 switchbacks, and shortened the route by 10 kilometers. Even so, the mountain still wins in winter. Heavy snow at both portals and the constant threat of rockfall can close the crossing without warning.

The pass owes its other name to something travelers rarely see anymore. Higher up, near the old Uspallata Pass at 3,832 meters, stands the four-ton bronze statue of Christ the Redeemer of the Andes, raised in 1904 to mark peace between Argentina and Chile. For decades that statue marked the highest point of the road, where every crossing had to climb. The 1980 tunnel changed all that, dropping the route hundreds of meters below the old summit. The monument still presides over the heights, but the modern flood of traffic now passes unseen beneath it, through the dark of the tunnel.

Long before the trucks, there were trains. The Transandine Railway crossed almost this same gap at the Uspallata Pass, opening fully in 1910 after nearly two decades of construction by the Clark brothers of Valparaiso. It was an engineering marvel of its day, using a toothed rack rail to grip the steepest grades and climbing to around 3,200 meters at its own border tunnel. The line collapsed the journey between the ports of Buenos Aires and Valparaiso from an eleven-day voyage around Cape Horn to roughly thirty-six hours by rail. Yet it was never a commercial success, hampered by limited capacity and the rise of motor traffic. The last freight train ran in 1984, and today travelers can still glimpse its abandoned tunnels, bridges, and snowsheds beside the modern road - the rusting bones of an earlier dream of crossing the mountains.

A single high tunnel is a fragile lifeline for two national economies, and planners have long dreamed of escaping the snow line. Proposals come and go: a John Paul II Tunnel bored lower at around 2,250 meters to link Horcones in Argentina with Juncal in Chile; a Las Lenas Pass crossing lower still, near 2,050 meters, joining the San Rafael region to Machali in Chile; even an ambitious 52-kilometer bi-oceanic railway tunnel beneath the entire range. For now they remain on paper. The trucks and buses keep climbing toward 3,200 meters, year after year, gambling against the weather at the roof of the continent.