An estimated 11-feet of sand swallowed a 6-year-old boy from Sterling, Illinois, for more than three hours before he was rescued in 2013.
A study concludes that fungi on the covered trees formed a sort of cement that allowed the sand to keep its hallowed out shape.
A popular sand dune will be indefinitely closed after a child was trapped in the sand last summer.
The Mt. Baldy area will remain closed to the public until testing and analysis of the hole are complete.