Environmental Protection Agency scientists use ground-penetrating radar equipment at Mount Baldy at the Indiana Dunes National Lakeshore near Michigan City, Ind. They are gathering data to determine the safety of a northern Indiana sand dune that collapsed onto a 6-year-old Illinois boy, burying him under 11 feet of sand. (Brian Schlender/News Dispatch via Associated Press)

Geologist Erin Argyilan has in the past 10 months studied sediment, analyzed wind patterns and mapped terrain, but she hasn’t solved the mystery of the holes that appear and vanish in the beige sands of Mount Baldy at Indiana Dunes National Lakeshore.

“We’re seeing what appears to be a new geological phenomenon,” she said.

Argyilan is one of many experts who have combed the terrain for clues about the origin of these potentially dangerous holes, which are about a foot in diameter and seem to survive for less than a day before filling in naturally with sand. Investigators have used ground-penetrating radar and specialized GPS devices to peek below the landscape, but no one is certain why the holes form along the surface of this landmark, which attracts thousands of visitors each year.

The National Park Service announced this week that Mount Baldy will remain closed indefinitely because of the discovery of two new holes and a number of depressions on the north side slope, although the rest of the Indiana Dunes National Lakeshore will be open.

Argyilan is driven professionally and emotionally to try to help explain this curious occurrence. The geology professor had been conducting research at the base of Mount Baldy in mid-July when she heard the screams of a couple about 400 feet away, frantically pointing at the ground that swallowed their 6-year-old son.

She initially thought they were mistaken, because there should be no holes in a dune, which is comprised of grains of sand with no room for air. But when rescue crews saved Nathan Woessner of Sterling, Ill., after a three-hour burial under 11 feet of sediment, it simultaneously shattered the scientist’s understanding of the terrain, and hurt her emotionally.

“I cried for three days,” said Argyilan, associate professor of geoscience at Indiana University Northwest, who was seven months pregnant at the time and couldn’t stop replaying in her mind the horror of the day. “I couldn’t help in the moment. So now I have to do what I can to learn why this is happening.”

In August, a second hole was spotted nearby, this one about 10 inches wide and at least 5 feet deep, prompting further study.

One theory is that decaying trees beneath the sand might be causing the holes to form.

Human activity on the dunes and along the shore has led to the erosion of Mount Baldy. The 126-foot dune has been shifting away from the lakeshore at an alarming rate, covering the old landscape in sand, experts have said. The thought is that cascades of sand drifting south are burying trees, which could be rotting and creating the holes beneath the surface, Argyilan said.

While nothing is certain, she believes that holes are unlikely to start appearing at other sand dunes that have been kept in a more natural state.

“This particular dune has a complicated history,” she said.

Historical mining of the sand to make glass Mason jars — as well as the existence of man-made structures such as jetties on the harbor — could have contributed to the erosion problem, Argyilan said, adding that she has evaluated pictures of the landscape from 1938, and the changes are drastic.

Most sand dunes are covered in vegetation compared with Mount Baldy, where the trees and grass — and even an old staircase that once led to a viewing spot — are now buried, Argyilan said.

During the closure, the National Park Service will be planting new grass where the native vegetation once grew in the hopes that it will hold the sand in place and perhaps prevent new holes from forming.

Argyilan said that the holes in Mount Baldy are distinct from the “sinkholes” that typically consume cars and homes. These are usually from rock surfaces that dissolve over time because of water damage.

National Park Service officials say they’re preparing for a more thorough investigation of the dune this summer, including mapping of holes and depressions, use of special ground-penetrating radar and coring of samples of sand and organic matter. The Environmental Protection Agency has used ground-penetrating radar to identify anomalies beneath the surface, but experts aren’t certain what caused the patterns.

— McClatchy-Tribune