On a perfect summer day in 2013, 6-year-old Nathan Woessner raced after his father across the shifting sands of Mount Baldy dune, nestled on Lake Michigan’s southern shore. But in an instant, fun turned to terror when Nathan disappeared into a gaping void in the sand, 12 inches wide and 11 feet deep.
At the time, speculation flew about the cause of this hole—everything from lightning and hole diggers to buried abandoned houses caving in, Ariel Sabar reported earlier this year for Smithsonian magazine. As evidence rolled in, many scientists suspected that these holes were casts of rotting trees that the rapidly migrating Mount Baldy entombed in its sandy mass, writes Sabar.
But how exactly these holes remained open as the wood rotted away remained unknown until now. According to new research presented this week at the Geological Society of America conference, the answer is fungi.
At the time of the boy’s disappearance, rescuers observed some key features about the tube, Sabar writes. "Its inside wall was sandy and soft, but bore the imprint of bark, almost like a fossil. It was as if the boy had wound up at the bottom of a hollowed-out tree trunk, except not a bit of tree was there."
So scientists set out to get a closer look at the network of holes. They used all manner of equipment to locate the tunnels, from paint brushes and trowels to ground penetrating radar, discovering evidence of decaying trees, according to a press release.
"At that point, I was sold that we had trees being buried and decomposition driven by fungus," lead researcher and coastal geologist Erin Argyilan says in the release . "But I did not know why the holes would stay open."
Scientists collected sands from the tunnel walls to study using high-powered microscopes, which revealed that the fungus that likely consumed the woody mass of the trees extended "roots" or hyphae into the sandy walls. Along with cementing minerals, the hyphae held the dry sands in place, preventing their collapse.
Argyilan is now turning investigation towards the cement, which she believes is a byproduct of the combination of decomposing wood and weathering of the sandy walls.
As for Nathan, rescuers found him just in time, digging a massive hole to retrieve the cowering boy from his sandy cavern. But, as Argyilan told Sabar earlier this year, Mount Baldy’s tunnels are a startling reminder that there remain many geologic mysteries yet to be solved.