MICHIGAN CITY, Ind. (AP) — Trees slowly buried by windblown sand are likely the root cause of dangerous holes that have appeared in a towering sand dune on the Indiana Dunes National Lakeshore.

The South Bend Tribune reports that geologist Erin Argyilan believes pockets were formed around tree branches and trunks that were enveloped by the shifting sands of Mount Baldy. She says that created a “ghost forest” beneath the sand.

The popular tourist destination was closed in 2013 after a boy from Illinois became trapped under 11 feet of sand before being rescued. Scientists have been searching for the cause ever since.

Argyilan says 11 holes beneath the sand have been discovered and she is certain more exist. According to the National Park Service, Mt. Baldy moves about 4 feet a year.