Only the bristlecone pines of California are known to live longer. The alerce, Fitzroya cupressoides, is the second-longest-living tree species on Earth, with one Chilean specimen, the Gran Abuelo or Great Grandfather, verified by counting growth rings at more than 3,600 years old. It grows agonizingly slowly, sometimes only a millimeter or two of girth in a year, which is precisely why its dense, rot-resistant timber became so prized, and so endangered. For generations loggers felled trees that had outlasted entire civilizations, sawing centuries into shingles and fence posts. Argentina drew a line in 1937, creating this park specifically to protect its alerce forests. Today the largest such stand in the country survives here, a relict of a wetter, older world. Often likened to California's giant sequoias, the alerce is rarer still, clinging to a narrow band of southern Chile and Argentina and growing nowhere else on the planet. The endangered groves are now among the most carefully guarded forests in South America.

Glaciers shaped everything you see. As the ice sheets advanced and retreated across millennia, they gouged out cirques, piled up moraines, and left behind a string of impossibly clear lakes. The park is essentially one long watershed, a chain of lakes linked by short, turbulent rivers, all of it eventually draining toward Chile. Lake Rivadavia spills into Green Lake; the Arrayanes River threads onward to Futalaufquen Lake. Above the tree line at about 1,400 meters, the forest surrenders to bare rock and snowfields beneath peaks like the Cordón de las Pirámides, the park's high point at 2,440 meters. The Torrecillas glacier still hangs between the two arms of Lake Menéndez, visible from the tour boats that ferry visitors toward the ancient groves.

This is a park of dramatic contrasts written in rainfall. Moisture-laden clouds roll off the Pacific and slam into the Andes along the Chilean border, dumping up to 3,000 millimeters of rain and snow a year on the western forests. These are Valdivian temperate rainforests, lush and dripping. But just a few kilometers east, in the mountains' rain shadow, that figure collapses to around 800 millimeters, and the landscape thins into drier Patagonian woodland of coihue and lenga. Freezes can strike in any month. Along the Arrayanes River, cinnamon-barked arrayán trees lean over the water, their smooth orange trunks glowing against the green.

In 2017, UNESCO inscribed Los Alerces as a World Heritage Site, formal acknowledgment that these forests represent one of the last large tracts of continuous Patagonian forest left in a nearly pristine state. The park is vast, more than 250,000 hectares counting its adjoining reserve, yet almost all the roads, trails, and lodges are confined to a thin developed strip, leaving the heart of the forest essentially untouched. Protection, even so, has never been simple. A hydroelectric dam flooded three natural glacial lakes to create the Amutui Quimey Reservoir, generating power for distant industry on the Atlantic coast. The tension is the same one that runs through all of Patagonia: a place this wild is also a place full of water, timber, and energy that someone, somewhere, would like to harness. For now the alerces stand. Boats still cross Lake Menéndez to Puerto Sagrario, where visitors walk softly among trees that were already ancient when the first ships reached the Americas, in a silence broken only by wind and water.