SOUTH BEND, Ind. (AP) — Researchers say mysterious holes that forced the closure of a massive dune at an Indiana national park after a 6-year-old boy fell into one and nearly died were caused by sand-covered trees that left cavities behind as they decayed over the years.

A study published in December and a second due out this summer that supports its findings determined that fungi on the covered trees formed a sort of cement that allowed the sand to keep its hollowed out shape as the wood decayed and collapsed inward.

Bruce Rowe, a spokesman for the Indiana Dunes National Lakeshore, says the studies will determine whether the popular dune, Mount Baldy, can be reopened. Researchers say the phenomenon is likely responsible for holes found in migrating dunes in Oregon and Michigan.