WE ARRANGED TO RENT a Land Cruiser in a dimly lit office in Uyuni, arriving at 6 p.m. and hoping they could have it ready by six the next morning. Time was running out for Criso and Maria, who we were told would pilot the jeep, to buy fuel and supplies, but Greg was meticulous, almost rudely so, reading a checklist out loud and asking repeatedly about water, food, spare tires, hotels along the way, what the food was, what the hotels were like, how many spares there were, how well Criso knew his vehicle, etc.

"I've had problems with Land Cruisers in Tibet," he explained. He was agitated, as if the closer he got to the confluence, the more it weighed on him, the more he wanted to control the variables. In the morning, he got up at 5 A.M., an hour early, waking himself just to organize his backpack and ensure all his gadgets were in order. He was so thorough that he was still the last one ready.

Between Uyuni and the confluence is the world's largest salt flat, some 65 miles by 65 miles. The Salar de Uyuni sits at 12,000 feet, is shaped like an amoeba, and is filled, unsurprisingly, with salt: man-made salt mounds, a salt hotel, a salt highway, saltwater springs, and blinding salt-pan views from cactus-dotted "islands." We rolled onto it at dawn, and Greg was serene again, happy about Criso and Maria's (short-lived) familiarity with our surroundings. We stopped briefly at a salt-mining operation. We ate breakfast at the salt hotel, at a table made of salt. Undisturbed, the Salar's perfectly white surface had dried to form a mosaic of interlocking hexagons that stretches miles across the emptiness, and in the distance we sometimes saw mirages or speeding trucks whose hum sounded like a jet taking off. Greg bent over to lick one of the ridges "just to make sure" it was salt. It was.

Our route took us less than a mile from 20°S 68°W, low-hanging fruit that had been bagged twice before. Perhaps for the benefit of Paolo and me, Greg decided we should go for it anyway, so after lunch he and I pulled out our GPS units and watched the numbers tick down. On Greg's wrist was an altimeter/compass watch. He had the Google Earth screen shots on his music player as well as printouts of the same, plus photocopies of some military topos. He had a backup GPS, a backup camera, and a backup compass in his bag. Combined with the arsenals Paolo and I have, this brought us to a total of three compasses, four GPS units, six cameras, and perhaps three dozen maps. We were ready for action, and I could not deny the excitement of the moment when it came.

"Tell him to slow down and go to the left," Greg told me, and I relayed his message in Spanish. Criso veered off the track, and the hum of our wheels quieted. My GPS showed the distance dropping rapidly: 1.35 kilometers, 900 meters, 250 meters, 45 meters. "Now, now, now … Stop!" Greg yelled. We threw open the doors. It was windy out and very cold, and the salt crunched underfoot as I waved my GPS back and forth, trying to follow it in.

"Now I usually get out my compass and try to figure out where I need to go to make the zero," Greg said. "The GPS read .007, which meant we were a little south." We strode north, then slightly east, and my newer GPS locked it in: 20°00.000'S, 68°00.000'W, accuracy plus or minus three meters. "Photograph it as quickly as possible," Greg advised. "It'll change." He lurched back and forth, trying to get his own GPS to zero out—the "confluence dance." Soon he hit it, too, and grabbed for his camera. We snapped photos in the four cardinal directions—of salt, salt, salt, and salt, respectively. Greg pulled out a pad and scribbled some notes. He smiled. We were done. As we left, he took a one-boliviano coin from his pocket and placed it on the confluence—a gift for any brave explorers who followed.

Thus baptized, we drove out of the Salar and into the unknown, following a web of dirt roads toward Sajama, stopping for directions in almost every windswept village, relying more and more on the GPS and map. In one village, Llica, Criso asked the guys at an auto shop which direction we should go. Straight, they said. We passed a family of quinoa farmers working a barren patch of dust, Maria hopped out, and we all watched as they pointed back the way we'd come.

We found our way across a smaller salt flat, then rumbled through the Altiplano, passing deserted villages, sand dunes, and Stonehenge-like clusters of rock tombs, called chullpas, which Maria said are filled with the bones of an ancient race of midgets who were killed by the sun. We picked up—and got directions from—a hitchhiking grandmother and her four grandchildren, backtracked out of the bog, talked down some soldiers who wanted a bribe, got lost and found our way a half-dozen times, and overnighted in a truckers' hostel in a run-down border town. Now, on the afternoon of the second day, we've finally reached Sajama National Park, where we see a herd of alpacas grazing on the flanks of the namesake volcano.

Opposite the volcano is the confluence peak—we recognize it on sight—and Greg has a final request for Criso and Maria: that we use the remaining daylight to detour toward the peak and scout our line. They bristle. "Our job was to take you to Tomarapi," Criso says.

"No, no, no—we need to get as close as we can," Greg says, his voice tense. "It's so ridiculous. Of all the out-of-the-way stuff we did today, we can't do this? This is the most important thing."

They go silent and make long faces, hoping he'll relent. He doesn't. Just before Toma­rapi, we turn off and drive most of the way up the jeep track and stare at our destiny. It looks more or less like it did on Google Earth. Even so, we use Paolo's long lens to shoot close-up photos of the confluence, the volcanic bulge it sits upon, and the cliffs guarding the approach. That night, after a quinoa-and-soup dinner at the eco-lodge, we pull out my laptop, upload Paolo's photos, and compare them with maps and printouts until we're certain our ridge route is best. Paolo and I go to bed early, but Greg stays up late, shuffling and organizing, readying his pack, his cameras, and his multiple GPS units.