Greg tells me this as we stand, lost, in a village in western Bolivia, surrounded by alpaca droppings and bicycle tracks and adobe huts with straw roofs and cactus-wood doors. Our location is 18°50.983'S, 68°31.233'W—cer­tainly nothing special, not for a man of Greg's stature—and we're just miles from the border with Chile, which is marked by a reddish, perfectly conical volcano. Ahead, across the Altiplano plateau, are the glaciated peaks of Sajama National Park, home to the highest confluence point in the Western Hemisphere. We think we can see the mountain we'll have to climb to reach it, but we can't be sure, and in any case it's 65 miles away. First we have a bog to negotiate.

Our driver, Criso Ibieta, and cook/navigator, Maria Garcia Medina, clearly have never been here. Since yesterday afternoon, they've been bickering about directions and relying heavily on photographer Paolo Marchesi's Bolivia map and on my new GPS, which now sits between them in an honored spot on the front seat of the Land Cruiser. Soon there are two dirt tracks to choose from; we go with the one that heads straight toward Sajama, bouncing along for a mile until it dead-ends at a river in a broad, soggy meadow. Beyond the river are sand dunes, more volcanoes, and hundreds of alpacas. We get out and walk up the banks, trying and failing to find a place to cross. We stare at the volcanoes. Greg stops to snap a photo. He's sporting sunglasses, a soul patch, and a pair of those zip-off travel pants that convert into shorts—looking, as always, about a decade and a half fitter and younger than his 39 years.

"We'd probably be on some tourist path if we didn't have this mission," he says. "You might think I get a little carried away, and some people say that I am, but most of the world has been explored. This is a measured way to assure that we visit all the in-between spaces—that we see what's there. Confluence hunting is the last frontier."