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MICHIGAN CITY | Officials at Indiana Dunes National Lakeshore announced Thursday that scientists still do not know what caused holes to appear in Mount Baldy last summer, and the popular attraction will remain closed for further study.

Nathan Woessner, 6, of Sterling, Ill., was swallowed by a hole July 12 and rescued by firefighters.

Two additional holes have appeared since July, park officials said Thursday.

Ground penetrating radar studies performed by the Environmental Protection Agency have identified a large number of anomalies below the dune’s surface, but scientists from the National Park Service, Indiana University and the Indiana Geological Survey still do not know how these holes were formed.

“Mount Baldy is one of the most visited sites in the national lakeshore, attracting thousands of visitors each year” said Acting Superintendent Garry Traynham in a press release. “But the continued development of these holes in the dune surface poses a serious risk to the public. Our first obligation must be to the welfare of our visitors who are here for an enjoyable outing.”