Perito Moreno is a wall of ice five kilometres wide standing seventy metres out of a lake in Argentine Patagonia, and it breaks pieces off itself all day long. The sound comes first, a crack like a gunshot, and then a tower as tall as a building leans out and goes into the lake. What it leaves behind is a raw blue scar. The blue is not a trick of the light. Centuries of snow have been pressed into ice so dense that the air is squeezed out, and airless ice throws back blue, brightest on a face that has just calved. Steel walkways run the length of the front on several levels, four kilometres of them in all. The lower ones bring you nearly level with the water; the upper ones pull back far enough to take in all five kilometres.
Nothing announces a calving, so most of what you do here is sit still and wait for it. The walkways face west, so the morning sun lands full on the ice and lifts the blue out of it. Boats cross to the southern face and hold out from the wall, which from the deck stands over you; a calving nearby sends its swell under the boat. It is also one of the few glaciers anyone can walk on, crampons on your boots, out among blue crevasses and meltwater channels, and the short routes end with a whisky poured over glacier ice. Quieter paths run through the lenga and ñire forest along the shore, the ice flashing between the trunks; in April the beeches turn red. The glacier used to do something bigger: every few years it pushed across the southern arm of the lake and dammed it. The water behind rose, floated the front off the rock, and the span left above fell in front of crowds. That may be finished: thinning and pulling back into deeper water, it is not expected to reach the far shore again. Everything else it does, it does daily.