Perito Moreno is a glacier in the Andes of Argentine Patagonia whose front stands five kilometres wide and seventy metres high in the waters of Lake Argentino, and it breaks off pieces of itself all day in front of boardwalks laid along the facing shore. Unlike most glaciers it is not retreating; its face pushes into the lake at up to two metres a day, which is why it is always calving. The sound comes first, a crack like a gunshot, and then a tower of ice the size of a building leans out from the wall and drops into the lake, leaving a raw blue scar. The blue is not a trick of the light: the ice is so old and so compressed that all the air has been squeezed out, and dense ice absorbs every colour except blue. Steel walkways run the full length of the front at several levels, and the thing to do is work the whole circuit rather than stand at the main balcony, because the calving comes without warning.
Boats cross the lake to the southern face and hold position a safe distance out, where the wall towers over the deck and the water is full of floating blue fragments. Perito Moreno is also one of the few glaciers on Earth you can walk on: guided treks strap crampons to your boots and lead you out onto the surface, among crevasses, meltwater channels and pools of a blue that looks poured. Away from the balconies, quieter paths wind through the lenga and ñire forest along the shore, with the ice flashing white between the trunks. Every few years the glacier reaches the opposite shore and dams the lake's southern arm, the water behind it rises by metres, and then the trapped water floats the ice and bursts through it in a collapse that draws crowds from across the country. The name is not a place but a job title: perito means expert, after Francisco Moreno, the explorer who surveyed this frontier, and the glacier that carries it is the most reliable spectacle in Patagonia.