Two million years ago, it wasn't safe to go in the water, thanks to a predator the size of a Greyhound bus. These days, you have to search the Peruvian desert to get face-to-face with the granddaddy of Jaws.

"Roberto, Roberto, come see!"

I'd made a remarkable discovery in the Ocucaje Desert, 12 miles inland from Peru's southern coast, but my guide was a quarter of a mile away, relieving himself in a gully. I jumped up and down, screamed, and did a few cartwheels, which made my head swirl, but still no Roberto. Here in one of the oldest and quietest places on Earth - a wondrous desert of tiered escarpments, sedimentary rock, and level stretches between Precambrian volcanic mountains - no one else would hear my squeals. Waiting for Roberto to reappear, I paced back and forth across a fossil-embedded­ steppe, a broken section of the ocean floor that was thrust 2,600 feet above sea level 12 million years ago. When he finally returned and saw my big surprise - a three-inch-long fossilized tooth from a megalodon shark - a beaming Roberto said, "Pachamama ["earth mother"] sent me to the water closet­ so you'd find this gift."

Three weeks earlier, I'd read an online posting about Roberto Penny­ Cabrera, and called him to find out more about the ocean's largest-ever predator. "Forget Jaws. The megalodon was more than twice as long as a great white shark, much larger than a Greyhound bus," he told me. "With an enormous mouth 11 feet high and nine feet across, it could break a whale in two with one bite." Roberto's enthusiasm for "sharkies" was endearing. Later I learned he was actually saying "shark teeth" so fast that the words ran together, but by then I'd grown fond of the term and chose to hear it that way.

A self-taught geologist and finder of prehistoric shark teeth, Roberto is the eyes and ears of world-­renowned paleontologists who share his obsession for the Carcharocles megalodon shark, which terrorized the sea between two million and 16 million years ago. Consumed with the geology of the Ocucaje Desert, this 50-year-old with an aristocratic Spanish heritage prefers a life of few possessions - some desertworthy garb and gear, books, a chess set, and a signed declaration from Peru's National Institute of Culture that names him an official protector of the Ocucaje. His home in Ica, 200 miles south of Lima, is decorated with satellite maps of the desert. Most of his megalodon collection is stored elsewhere - perhaps someday he'll open his own museum. Even so, Roberto considers himself a finder, not a collector.

I FIRST MEET ROBERTO a few miles from Ica at my hotel in Huacachina, a tiny palm-fringed lake surrounded by monstrous sand dunes. "So, how'd you hear about me?" he asks. "And why would a woman from New York City want to go 150 kilometers off-road to look for sharkies in one of the driest deserts in the world?" I blurt out: "When I was a little girl, my grandfather used to take me to a canyon in California, where we found fossils of fish and seashells. I was completely in awe of the fact that something millions of years old could still exist. My grandpa was the most fascinating man I'd ever met." Upon hearing this story, Roberto melts. And suddenly I realize that my entrée to the depths of the Ocucaje hasn't been a sure thing. "You've passed the test," he announces. "I will pick you up tomorrow morning at eight."

Only then does Roberto pull out a map and an album. "Gail, what you're about to see will shock you. If I showed these photos to everyone in Huacachina, there'd be a line of people fighting to go with me." I am mesmerized. We're talking fossilized whale skeletons, giant shark teeth larger than a man's hand, pre-Hispanic pottery partially­ exposed in the sand, and human bones and skulls exposed by grave robbers, or huaqueros, who plundered the 3,000-year-old tombs of the Paracas people.