The ice field does not sit still — it bleeds outward in every direction, feeding dozens of glaciers that grind down toward the sea on both sides of the divide. To the west, rivers of ice tumble into the labyrinth of Pacific fjords. To the east, they spill into the great Patagonian lakes Viedma and Argentino, whose meltwater eventually reaches the Atlantic through the Santa Cruz River. The roster of glaciers reads like a catalog of giants: Viedma, Upsala, Grey, Tyndall, O'Higgins. Largest of all is the Pío XI, also called the Bruggen Glacier — at some 1,265 square kilometers, the biggest and longest glacier anywhere in the Southern Hemisphere outside Antarctica. A single ice field gives birth to all of them.

The most famous of its children is the Perito Moreno, in Argentina's Los Glaciares National Park, and it is famous for breaking the rules. While glaciers across Patagonia and most of the world are retreating in a warming climate, Perito Moreno is one of only a handful still holding its ground, advancing at roughly two meters a day. Its true spectacle comes when its creeping front bridges an arm of Lake Argentino and dams it, raising the water on one side by tens of meters. Pressure builds for years until, with a roar that draws crowds from around the world, the ice arch shatters and the trapped water bursts through. It is one of the great natural performances on the planet — a wall of ice collapsing on cue, more or less, every few years.

For all its frozen stillness, the ice field hides fire. Two volcanoes lie buried beneath the plateau — Lautaro and Viedma — among the least studied volcanoes in either Chile or Argentina, simply because reaching them means crossing some of the most hostile terrain on Earth. The ice that conceals them also protects their secrets. Much of the surrounding wilderness is shielded inside great national parks: Bernardo O'Higgins and Torres del Paine on the Chilean side, Los Glaciares on the Argentine. Within those boundaries the ice field remains one of the planet's last genuine blank spaces, a place where geography is still being filled in rather than merely admired.

Humans have only ever scratched at the edges of this place. Serious exploration came late — expeditions led by Federico Reichert before the First World War, by Alberto de Agostini in the 1930s, by Eric Shipton in the early 1960s. From the air, the German aviator Gunther Plüschow made the first flights over the ice in 1928 and 1929, and a glacier now bears his name; during the Second World War, American military aircraft photographed it from above at Chile's request. Yet the first full north-to-south crossing on foot was not achieved until 1998, by a four-man Chilean team. More than a century after explorers first probed its margins, large stretches of the interior remain, in the truest sense, unexplored — too cold, too broken, too remote to give up their last secrets easily.