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."