Clad in hiking boots, a khaki shirt, an Australian oilskin hat, lightweight pants with zip-off legs, and with a 10-inch knife hanging from his belt, Roberto arrives the next morning driving the heavily­ modified 1981 Datsun truck he fondly calls Hermelinda. Provisioned with food and bottled water, camping gear, firewood, three spare tires, and a makeshift shower with a 27-gallon tank, we are good to go. We take off, first through the little town of Ocucaje, where locals we pass acknowledge Roberto's desert foray with a salute. We won't see another human being for two days.

Within an hour, we're off-roading in a landscape formed over millions of years by colliding tectonic plates. Our first stop: a large fossilized whale skeleton that includes eye sockets, a skull, and a partial spine and vertebrae. So what happened to the rest? "Erosion wouldn't destroy half a whale," Roberto says. "No, the lower part had to be taken by something strong, and that's the megalodon. See the sharp break in the spine? Sometimes you'll even find teeth marks on the bones." All around us are earthy-looking heaps, more fossilized skeletons, but Roberto is barely interested in the whales - all he can think about are megalodons.

As Hermelinda bounces us deeper into the Ocucaje and temperatures soar to 96 degrees, ­caffeine-addicted Roberto guzzles quarts of warm Coca-Cola - diesel for his body, he declares - and I chug bottled water. When the heat gets to him, Roberto stops to take a shower fully clothed and climbs back into the truck, dripping wet. Every couple of hours, he checks in with one of two SOS contacts to make sure someone knows our whereabouts at all times.

Periodically, he makes a close-up inspection of an area before promising that shark teeth can be found there. What is he looking for? "First, you must find a spot with brown," he says, fingering some powdery rust-colored soil, "organic remains, mostly plankton. And when you have plankton, you'll find a chain of consequences of life - fossils, shells, small pieces of bone, and also shark teeth. But you won't see the teeth unless a strong wind hits the ground at just the right angle to bring them out into the open." Taking them out of the desert is entirely legal, I learn, because within 10 to 12 weeks, wind and sun will break them into little splinters.

Roberto spots a dozen teeth before I find my first, an inch-long mako shark tooth. I quickly realize this sport can be addictive - we don't even stop for lunch. By late afternoon, I've found only a few small teeth, but Roberto reassures me. "Don't worry, Gail. We're just starting today. Tomorrow you'll find the big one. Now it's time for some dinner."

We set up camp in a sheltered spot below a six-foot-high ledge ofdiatomite, a sedimentary rock that's rich in ocean planktonremains. Within minutes, Roberto unloads lounge chairs, sleepingbags, a small table, firewood, and the food - fresh-baked rolls assoft as cotton balls, plus cheese and assorted canned goods.Famished, I grab a can of frijoles, which Roberto opens with hisknife, and I eat them straight from the can without even botheringto heat them up. As I wolf down my meal, I realize howextraordinary it is to be dining beneath a 23-million-year-oldpiece of the ocean floor, and I think that perhaps the remarkablesurroundings help make the beans so sweet and tasty, despite theirbeing cold. Later, Roberto builds a campfire and pours boxed redwine into our mugs - he spikes his with Coca-Cola (more diesel) andthen we toast: "Salud! Mañana, el megalodon."
For a while, we stop talking about sharks, or anything else forthat matter, indulging only in the delicious absence of sound. Andwhen our campfire burns down to a few glowing embers, a moonlessdark sky graces us with a larger-than-IMAX screening of a meteorshower.