The mystery holes that plague an Indiana sand dune—and triggered its public closure when one in July 2013 swallowed a 6-year-old boy who nearly died—are apparently caused by a "ghost forest" of underground trees and "cement." Per a recent study, buried trees at Mount Baldy dune are decaying in such a way that gives the holes—the former branches and trunks—a temporary structure, the AP reports. As for how the hollow shapes hold that structure, scientists point to a "calcium-carbonate-rich cement" found "at the contact between organic material [the decaying trees] and sands." In the sand, parts of "decayed trees progressively collapse and infill, and open holes are temporarily stabilized" by the cement, the researchers wrote in Aeolian Research in December. "Further, holes can exist undetected at the surface, covered by a thin veneer of sand."
Lead author Erin Argyilan is working to figure out what's causing the "cement," which may flourish when tree-decaying fungi comes in contact with sand. "It’s not in the sand. Is it the tree itself? Is it the fungus? The sediment? What is the key factor that is making this happen? The work we are doing now will show ... how the materials got there," she tells Indiana University Northwest News. She's also involved in a study that will create a map of possibly dangerous areas at the dune using 1930s photos that show where the trees stood before the migrating sands—the dune moves an average 10 to 13 feet a year—buried them. No one's saying whether Mount Baldy at the Indiana Dunes National Lakeshore will reopen, but there may be a bigger question: "There are actually a lot of places around the country ... where dunes are covering trees," says Argyilan, whose findings could be used to assess risk elsewhere. (In California sand dunes, archaeologists found a giant sphinx.)