Geologist Erin Argyilan has in the last 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.

She is one of many experts who have combed the terrain for clues on the origin of these potentially dangerous holes, which are about a foot in diameter and seem to last 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 entirely certain why the holes form along the surface of this landmark, which attracts thousands of visitors each year.

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

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

She initially thought they were mistaken because there should be no holes in a dune, which consists 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 geosciences at Indiana University Northwest, who was seven months pregnant at the time and couldn't stop replaying in her mind the horror of that 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, that one about 10 inches wide and at least 5 feet deep, prompting further study.

One theory is that decaying trees beneath the sand might cause 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.

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

Most sand dunes are covered in vegetation, unlike 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 closing, the National Park Service will plant new grass where native grass once grew in the hope that vegetation will hold the sand in place and perhaps prevent new holes from forming.

Argyilan added that the holes in Mount Baldy are distinct from the sinkholes that typically make headlines for consuming cars and homes. Those usually develop from rock surfaces that have dissolved 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 holes and depressions, using ground-penetrating radar and coring sand and organic matter for samples. The Environmental Protection Agency has used ground-penetrating radar to identify anomalies beneath the surface, but experts aren't certain what caused those patterns.

"We don't know exactly what's going on out there. We can't let folks out onto the area until we know it's safe," said Ken Mehne, law enforcement specialist for Indiana Dunes National Lakeshore.

He said rangers will enforce the closing, which will likely last through the summer and possibly into the fall. The rest of the National Lakeshore will remain open, though officials request that visitors stay on designated trails to prevent erosion and damage.

"There are 14.5 miles of other beach areas in the park," Mehne said. "There are plenty of other places open if folks want to come out to the park."

eleventis@tribune.com