Erin Argyilan was wrapping up a scientific study of wind speeds on Mount Baldy last year when she saw a circle of beachgoers on their knees halfway up the hulking sand dune. They appeared to be digging frantically.

It had been a gorgeous afternoon: sunny, mid-70s. All day, a breeze had rolled off Lake Michigan and up the dune’s rumpled face. Rising 126 feet off the beach, Mount Baldy is one of the tallest lakefront dunes in the world and the most popular attraction at the Indiana Dunes National Lakeshore, a national park that straggles for 15 miles along the industrial southern shore of Lake Michigan, between Gary and Michigan City, Indiana.

For many of the park’s two million yearly visitors, the grueling hike up Baldy’s slip-sliding slope—and the dead run down—is a rite of passage. But on that July afternoon, Argyilan, an athletic 38-year-old geoscientist at Indiana University Northwest, who was then seven months pregnant with her first child, sensed that something was amiss. She strode up to the site of the commotion and saw a man in swim trunks clawing at the sand. “He’s here,” the man kept saying. “He’s right here.” His wife, who appeared to be in shock, was calling out to God. Their 6-year-old son, they said, had vanished down a hole.

Argyilan saw no sign of an opening or even upturned sand, which you’d expect if someone had dug a hole. As for natural cavities, dunes aren’t supposed to have any. Unlike hard rock, which can dissolve to form caverns and sinkholes, dunes are just big piles of sand formed as wind stacks one grain atop the next.

“This doesn’t make any sense,” Argyilan told a pair of fieldworkers from the National Park Service, who’d been helping lug around her 45-pound wind meter. Someone had called 911, and soon police and firefighters were clambering over Baldy’s crest with shovels.

Argyilan, a former CrossFit trainer with a nose stud and shoulder tattoo, was no milquetoast. As Hurricane Sandy bore down the October before, she’d summited Baldy in ski goggles to record the erosional brawn of the winds and waves. The sand-laced 50-mile-per-hour gusts scoured the numbers off her surveying rod. But now, as park brass arrived to coordinate an emergency response, Argyilan kept a cool distance. She scanned Baldy’s taupe slopes, sure the boy was just hiding somewhere. At 6 p.m., almost two hours after his disappearance, she packed up her wind meter and drove home.

He’ll turn up, she told herself.

For dinner that night, Argyilan, her fiancé and her father went to a nearby Applebee’s. As they finished their meal, the restaurant’s TV screens flashed with news from Baldy: After a three-and-a-half hour search involving 50 rescuers and a pair of construction-site excavators, the boy was found a dozen feet beneath the dune’s surface. He had no pulse or breath at first, and his sand-encrusted body was ice-cold.

“I felt absolutely shattered,” Argyilan remembers. Everything she knew about geology—all the courses she’d taken, all the papers she’d read over years of study—told her this couldn’t happen. But her science had led her astray.

She sobbed on the way home, and spent a sleepless night on the couch, hunting online for any reports of similar cases. She scolded herself for not digging alongside the father. As a mom-to-be, she wished she’d tried to comfort the boy’s mother. Worst of all, though, was a recurring thought: “If they had listened to me, they wouldn’t have kept looking.”

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We live in an era when the robotic arms of unmanned spaceships can scoop sand on Mars, then phone home across millions of miles to tell us its chemistry. Yet here, in the well-traveled regions of Earth, on the very ground we walk, we are still being surprised by geologic mystery.

In California’s Death Valley, “sailing stones” cruise the desert floor under a locomotion that science struggled for decades to explain. In Siberia’s Yamal Peninsula, a patch of permafrost snapped earlier this year into a gaping crater, a previously unrecorded phenomenon.

Mount Baldy stands out even in this rogues’ gallery. It lazes not in some far-flung badlands, but in the temperate Midwest, on a popular beach an hour’s drive from Chicago.

People have turned a scientific eye on minerals since at least the fourth century B.C., when Theophrastus, a disciple of Aristotle, wrote his treatise “On Stones.” But because deserts are inhospitable and sand has scant commercial value, the subdiscipline of dunes had to wait until the 20th century to find a champion. Ralph Bagnold, a Cambridge-educated engineer in the British Army, explored the Sinai and Sahara on leave before drafting his 1941 masterwork The Physics of Blown Sand and Desert Dunes, which is still routinely cited in scientific journals.

Today a great deal is known about dunes: how wind launches individual grains of sand and piles them into parabolas, ridges and other distinctive shapes; how plants steady dunes, and how waves weather them; how a dune’s history and age can be deduced from certain properties of buried sand and soil. Riddles remain (the movement of small particles is a complicated, chaotic business), but one thing about dunes has never been in doubt: their solidity.

“For sand to accumulate in a way that would leave holes or caverns in the subsurface doesn’t at first glance make a whole lot of sense,” says Alan Arbogast, a Michigan State University geographer who is the leading authority on the region’s dunes.