SOUTH BEND, Ind. (AP) - 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, researchers have found.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, leaving holes more than 10 feet (3 meters) deep in the dune known as Mount Baldy at the Indiana Dunes National Lakeshore, according to a study published in December by Erin Argyilan, who heads Indiana University Northwest's department of geosciences.