CHARLES DARWIN STARTED IT. The Voyage of the Beagle, his 1845 account of spending five years as a naturalist aboard Captain Robert FitzRoy's ship, was the first advertisement for Patagonia, luring adventurers and outlaws to the bottom of the world. Butch and Sundance holed up in Cholila, Argentina, in 1902; in the early thirties, Antoine de Saint-Exupéry threaded his plane through the Andes' high passes, delivering mail; and Paul Theroux proved in his 1979 bestseller The Old Patagonian Express that the name itself exerts a due-south pull.

But it was the late British travel writer Bruce Chatwin who cemented the myth. His 1977 travelogue In Patagonia became a backpack staple, depicting an outpost of exiled Nazis and eccentric Welsh farmers living where the wind rattled "like an unloaded truck banging over a bridge." Some of Chatwin's hosts later said he'd made most of it up—a fitting charge for a place that shimmers on the frontier between geography and romance.

Patagonia encompasses most of the great tail of South America, from roughly 39 degrees latitude down across the Northern and Southern Patagonian ice caps, over the granite massifs of Fitz Roy and Torres del Paine, and on to the northernmost islands of Tierra del Fuego. On the Argentinian side of the Andes, the land is flat and dry; on the Chilean, clouds traveling 5,000 miles across the Pacific dump up to 200 inches of annual rainfall on a tangle of forests and fjords.

The result is the perfect stomping ground for a certain kind of rugged individualist. Iconoclastic climber Yvon Chouinard named his clothing company Patagonia, inspired by his 1968 road trip in a Ford van from California to 11,073-foot Fitz Roy. Over in Argentina, Ted Turner bought a monster estancia on a river full of monster trout. Tom Brokaw lit out for Chile as soon as he left his anchorman's desk last year.

So far, gringos have not loved Patagonia to death, but the wave is moving south. Rafters and kayakers and fly-fishermen have claimed the towns of Esquel and Bariloche in Argentina, the whitewater of Chile's Río Futaleufu, and, down by Fitz Roy, the Argentinian village of El Chalten. Next up seems to be the Chilean region of Aysén, an area four times the size of Vermont, with an emptiness reminiscent of America's West Coast 125 years ago: 91,500 people dispersed over 42,000 square miles, 72 percent living in the region's capital, Coyhaique. Aysén sees only 18,000 ecotourists a year, compared with 85,000 in Torres del Paine and 60,000 around the Fitz Roy massif.

And the prices! Ten thousand dollars for an acre and a half on 714-square-mile Lago General Carrera. Got a couple million? That'll buy you a fixer-upper in downtown Aspen—or your own little national park down here. One afternoon in Coyhaique, before we headed south to join Jonathan, I got the locals' tour from an expat Californian named Peter "Cado" Avenali, founder of Salvaje Corazon outfitters and cofounder of the Patagonia Foundation, an environmental nonprofit. Cado, 58, is the kind of strong-jawed adventurer who stays three steps ahead of the tourist stampede. We were driving southwest along the rapids of the Río Mañihuales; here and there, a broad black hat emerged from a dip in the landscape, rising to reveal a gaucho and a sturdy pony. Cado sat at the wheel of his Nissan pickup, trying to think of one bad thing.

"Well, there are no snakes," he said. "Nothing that bites. There's no poison oak. Wine's cheap and delicious. The lamb's great... ." He was stumped.

"You know, when I grew up in California," Cado said, "climbing at Camp 4 in Yosemite in the sixties, there was nobody there. And in 40 years, that's all gone. Patagonia represents to us what there was. In America, it's a good lifestyle and all, but I can't live with those restrictions. You start to feel like an animal—a big cat—with your back up against the wall.

"And there's more left," he continued. "Patagonia is an area that's the size, almost to the kilometer, of California, Oregon, and Washington, with a little bit of Idaho combined, and it has about 1.8 million people. It's like the old days of exploration. But there's no easy way to get to this stuff. You have to want it."

That's what made the Aysén Glacier Trail so alluring. We were headed off the map, it seemed, or at least out of the guidebook. Opening my copy of Lonely Planet's Trekking in the Patagonian Andes, I scanned the index for landmarks on the trail ahead. Puerto Bertrand, our starting point: nothing. Lago Colonia: no. The Neff Glacier: nada. It was as if they didn't exist.