How did a sand dune manage to swallow a boy?

It's been more than a year since a 6-year-old boy found a hole in a sand dune near the southern shore of Lake Michigan, began to lower himself in, and was swallowed several feet under for more than three hours.

"I'm scared," his father heard the boy say as family clawed at the hole. The boy survived, thanks it seems to an air pocket in the dune as well as a massive rescue effort, but the hole itself has left many geologists stumped.

Holes in sand dunes simply defy logic, reports the Smithsonian in an in-depth look at the episode and ensuing scientific search for answers. "One thing about dunes has never been in doubt: their solidity," writes reporter Ariel Sabar.

Erin Argyilan, a geoscientist who was doing field work at the dunes that day, has made it a mission to understand why this and other holes are opening up on Mount Baldy.

The mystery persists as she waits for test results on a prevailing hypothesis: that huge black oak trees—buried decades ago by the dunes' steady march inland—are decaying, leaving holes in their midst that reach to the surface in the shape of the branches that once were.

That the sand doesn't cave in could be attributed to chemicals released by the bark or a decaying fungus, prompting the sand to hold the shape of the vanishing trees.

Argyilan is now waiting for her samples to be tested at labs. "Mount Baldy is a great reminder that geological processes are still happening that are unrecorded," she says.

(Meanwhile, scientists are exploring another geological mystery in Siberia.)

This article originally appeared on Newser: How Did This Sand Dune Swallow a Boy?

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