Perito Moreno earned its fame by doing what almost no glacier does — for most of the last century, while its neighbours shrank, it kept grinding forward. Every few years the advancing ice drove right across an arm of Lago Argentino and dammed it shut. With no outlet, the trapped meltwater climbed as much as 30 metres above the main lake, until the pressure bored a passage through the ice and the bridge above collapsed in a roar heard for kilometres — a rupture first recorded in 1917 and repeated at irregular intervals ever since. The glacier covers about 250 square kilometres in Los Glaciares National Park, a UNESCO World Heritage Site since 1981, spilling off the Southern Patagonian Ice Field down to the lake. Its front runs five kilometres wide and rises some 70 metres above the water as a sheer wall of blue ice.
Most visitors take it in from kilometres of steel walkway built directly opposite the face, close enough to hear the ice crack and groan. Every so often a slab the size of an apartment block shears loose and slams into the lake with the report of a cannon. You can ride a boat to the foot of the wall, or pull on crampons and walk the glacier's broken surface on a guided trek. Lying 78 kilometres from the town of El Calafate, it is among the easiest great glaciers on Earth to reach. But the ice is changing: after decades as the exception, Perito Moreno has been retreating since about 2019, its northern front drawing back some 800 metres and its surface thinning many times faster than before. The lake-damming ruptures have stopped — the last full one came in December 2019 — and glaciologists doubt the glacier will ever cross the lake again.