Something strange is happening on one of Indiana’s sand dunes. Holes, about a foot wide and as much as 11 feet deep, are opening up in the sand on Mount Baldy at Indiana Dunes National Lakeshore, and then vanishing within a day.

The holes made themselves known in the worst way: One swallowed a six-year-old boy last summer, burying him under 11 feet of sediment for hours. He survived, but the incident prompted the National Park Service to close Mount Baldy to the public indefinitely. Almost a year later, at least two other holes have opened and closed, and 66 anomalies have been detected in the sand.

Video shows researchers using long sticks to poke at “soft spots”—depressions in the sand where holes could open up. Grains of sand disappear into the 126-foot tall dune as if being sucked into the bottom half of an hourglass. It’s enough to make one shiver, conjuring up terrifying thoughts of being buried alive. Scarier still: Researchers don’t know for sure what’s causing the holes.

"We're seeing what appears to be a new geological phenomenon," says geologist Erin Argyilan, who has been studying the holes for several months.

But there is a working theory. The dune currently sits on top of what was once a forest, and the thought is that trees buried beneath the sand for the last century are rotting and disintegrating, leaving holes where the trunks once were. "The age of the materials and the wet conditions during the spring of 2013 may have forced these materials to become unstable, collapsing and creating openings to the surface," says the park's website.

Mount Baldy is one of Indiana’s fastest-moving dunes. The wind causes it to move between 11 and 20 feet to the south each year, park ranger Bruce Rowe tells mental_floss. “We have seen evidence of trees that have been uncovered before as the dune moved on,” Rowe says. “But in the past they were always partially decayed stumps of trees. They were called ghost forests. They fell apart as they were exposed to the air. But the idea of holes rather than stumps is completely new to science.”

NPS Collection

It’s also baffling because, until now, dunes were considered solid structures. “We contacted various coastal dune specialists, and one of them said, ‘If I hadn’t heard this had actually happened from you, I’d say it was impossible,’” Rowe says.

So what’s to be done? This summer, a team of researchers will use ground-penetrating radar to map the dune’s thickness and identify potential openings at its base, and take core samples to identify materials within the dune. Dr. G. William Monaghan, a geologist and geoarchaeologist with the Indiana University Glenn A. Black Laboratory of Archaeology, says they also hope to use an unmanned aerial vehicle (a.k.a. a drone) equipped with lasers that can accurately map the area’s topography and identify changes. “...Or we’ll see if there’s a void down there,” Monaghan adds. “In a way we’re not sure what we’re gonna see until we’re out there seeing it.”

Until researchers get to the bottom of the mystery, Mount Baldy will remain closed. “This has become a really fascinating science mystery that’s playing out in our park,” Rowe says.