The glacier is one tongue of something far larger. It pours off the Southern Patagonian Ice Field, the great frozen reservoir that drapes the Andes along the Chile-Argentina frontier - the third largest expanse of ice on the planet after Antarctica and Greenland. Perito Moreno is one of 48 glaciers fed by that ice field, a river of ice roughly 30 kilometers long and 250 square kilometers in area. For decades it held a strange reputation among scientists: while glaciers worldwide retreated, Perito Moreno seemed to hold its ground, gaining mass nearly as fast as it lost it. The reasons for that long stability are still argued over by glaciologists, and recent years have brought signs that even this glacier may finally be giving way.

The glacier carries the name of Francisco Moreno, the Argentine explorer and naturalist whose obsessive surveys of this region in the late nineteenth century earned him the nickname Perito - the expert. It was no honorary title. When Argentina and Chile nearly went to war over where exactly their border ran through these uncharted mountains, Moreno's fieldwork became the backbone of Argentina's case in the 1902 arbitration that drew the line. He understood Patagonia as few outsiders ever had, and a glacier surveyed by a lieutenant of the Argentine Hydrographic Institute was named in his honor in 1899. The border he helped fix still runs across the ice field above, leaving a sliver of the glacier's origins in Chile and the rest in Argentina.

Here is what makes Perito Moreno extraordinary. The ice advances until its snout grinds against the Magallanes Peninsula on the far shore, sealing off a southern arm of the lake called the Brazo Rico. Cut off from the main body of water, the trapped arm has nowhere to drain. It rises - sometimes as much as 30 meters above the rest of the lake - pressing against the ice with mounting force. The glacier becomes a dam, and the dam cannot hold forever. Pressure tunnels into the base of the ice, carving an arch through the wall. The arch widens. And then it fails.

When the ice bridge collapses, the pent-up water of the Brazo Rico tears through in a torrent, draining back into the main lake and onward to the Santa Cruz River. In a sudden rupture the discharge can surge to thousands of cubic meters per second, and the spectacle draws enormous crowds - one event was watched by an estimated 10,000 people. The cycle is unpredictable, recurring anywhere from once a year to less than once a decade, and it does not always perform for an audience. The viewing zone closes at night, and more than once the arch has fallen in darkness with no one to see it, as it did in March 2018. There is no schedule. The glacier ruptures on its own terms, and you simply have to be lucky enough to be standing there when it does.

Up close, the glacier is not the silent white most people imagine but a fractured world of deep blue crevasses, meltwater grottoes, and seracs leaning at impossible angles. Visitors strap on crampons and walk directly onto the surface, choosing between a short mini-trekking circuit and a longer, more demanding traverse across the ice. And the ice is not lifeless. The Patagonian dragon, a tiny stonefly barely a centimeter and a half long, lives its entire existence on these glaciers, feeding on wind-blown bacteria frozen into the surface. Once thought extinct, it was rediscovered in 2001 in a recess of ice. On Perito Moreno, even the cold has its inhabitants.